Prove that if two miles are run in 7:59 then one mile MUST be run under 4:00.

Question

I'm in an argument with someone who claims that a two mile in 7:59 does not imply that one mile (at some point within the two miles) was covered in under 4:00. This is obviously wrong, but I'm not sure how to create a proof showing otherwise.

Does anyone have an idea for how to go about this?

Answer 1

If $x \geq 4$ and $y \geq 4$ then $x+y \geq 8.$

EDIT: on André's extra credit problem, use Beni's way of writing, time function $f,$ then define $g(m) = f(m+1) - f(m)$ with $0 \leq m \leq 1.$ We know $f(0) = 0, \; f(2) = 8.$ So, $g(0) + g(1) = 8.$ If both $g(0), g(1)$ are $4,$ we are done with André's problem. If one of the pair is above 4, the other is below 4. So, by the Intermediate Value Theorem, there is then some other $0 < m < 1$ such that $g(m) = 4.$

Answer 2

Proof by contradiction: If he/she had needed more than (or exactly) 4 minutes for the first and for the second mile, then he/she would have needed more than (or exactly) 8 minutes for both.

Answer 3

Consider when the runner passes the one-mile mark. If it is before 4:00 then he ran the first mile in less than 4 minutes. If it is after 3:59, then the second mile was covered in less than 4 minutes. But because 3:59 comes before 4:00 at least one of these cases (and possibly both) must be true.

Answer 4

You could model your case as follows: denote $f$ the time function, $f:[0,2] \to \Bbb{R}_+$, increasing, with $f(0)=0$, which shows the time at distance $x$. You know that $f(2)=7:59$.

If you suppose that $f(x+1)-f(x)\geq 4:00$ then $f(2)=f(2)-f(1)+f(1)-f(0) \geq 8:00$, and this is a contradiction.

Answer 5

I think the simplest way would be to prove it wrong by assuming that the person is right.

The person claims: 2 miles are done in 7:59, but each mile of those two miles is done in at least 4 minutes. If the person disagrees at this point, then you do not have to prove anymore. Otherwise you have two miles, each not less than 4 minutes. Therefore, in total it must be at least 8 minutes, which is against the assumption that two miles are done in 7:59.

Answer 6

The comment by André Nicolas is related to a very pretty theorem that deserves to be much better known. I first came across it in R.P. Boas's Traveler's Suprises, which appeared in The Two-Year College Mathematics Journal, 10 no. 2 (1979), pp. 82-88 (though I read it in the reprint that appeared in the highly recommended Lion hunting and other mathematical pursuits).

After giving the standard Mean Value Theorem problem ("if you travel in a differentiable way and your average speed is $50$ mph, is there some instant during which your instantaneous speed was exactly $50$ mph?") and a small variant ("Is there some small interval of time during which your average speed was also exactly 50 mph?"), he then gives two very similar but subtler questions:

1. If you travel for time $h$, more than one hour, and average 50 mph for the trip, is there necessarily some one continuous hour during which you covered exactly $50$ miles?

2. Suppose you run a considerable number $m$ of miles and average $8$ minutes per mile. Is there necessarily some one continuous mile (like a "measured mile" on a highway) that you covered in exactly $8$ minutes?

Boas provides the answers by using the Universal Chord Theorem. The theorem was first proven by Levy as a generalization of Rolle's Theorem in Sur une Généralisation du Théorème de Rolle, C. R. Acad. Sci., Paris, 198 (1934) 424-425.

If $f(x)$ is a continuous function, a horizontal chord of $f$ means a line segment with slope $0$ with both ends on the graph of $f$.

Here is Levy's version; you can easily see the similarity with Rolle's Theorem:

Theorem. The values $\alpha = 1, \frac{1}{2},\frac{1}{3},\ldots$ are precisely those for which the following statement is true:

If $f$ is a continuous, real valued function on the closed unit interval such that $f(0)=f(1)$, then the graph of $f$ has a horizontal chord of length $\alpha$.

Proof. Let $f$ be a continuous function defined on $[0,1]$, with $f(0)=f(1)$. Suppose $n$ is an integer greater than $1$, but that $f$ has no horizontal chord of length $\frac{1}{n}$. Let $g(x) = f(x+\frac{1}{n}) - f(x)$; then $g(x)$ is never zero on $[0,1-\frac{1}{n}]$, but is continuous, so it cannot change sign on that interval. If $g(x)\gt 0$, then $$\begin{align*} g\left(1 - \frac{1}{n}\right) &\gt 0\\ g\left( 1 - \frac{2}{n}\right) &\gt 0\\ &\vdots\\ g\left(1 - \frac{n}{n}\right) = g(0)&\gt 0 \end{align*}$$ which is equivalent to $$\begin{align*} f(1) - f\left(1 - \frac{1}{n}\right)&\gt 0\\ f\left(1-\frac{1}{n}\right) - f\left(1-\frac{2}{n}\right) &\gt 0\\ &\vdots\\ f\left(\frac{1}{n}\right) - f(0) &\gt 0. \end{align*}$$ Adding all of them up we conclude that $f(1)-f(0)\gt 0$, a contradiction. If $g(x)\lt 0$, a similar argument implies $f(1)-f(0)\lt 0$.

Therefore, $g(x)=0$ for some $x\in [0,1-\frac{1}{n}]$, but this implies that $f(x+\frac{1}{n}) =f(x)$; that is, the graph of $f$ has a chord of length $\frac{1}{n}$.

(Aside: You'll note this is exactly the argument Will Jaggy gives in his answer.)

Now assume that $\frac{1}{\alpha}$ is not an integer. Then Levy gives the following function: $$f(t) = t\sin^2(\alpha\pi) - \sin^2(\alpha\pi t).$$ Then $f(1)=f(0)$, but an easy calculation shows $f(t +\alpha) - f(t) = \alpha\sin^2(\alpha\pi)\neq 0$ for all $t$. $\Box$

If we assume that the travel in question 1 is continuous, and we assume that the travel in question 2 is such that time is an increasing continuous function of distance (no stopping, no backing up), then one can deduce the affirmative answer to both questions from the Universal Chord Theorem by using the same trick that one uses to deduce the Mean Value Theorem from Rolle's Theorem. I'll leave that derivation to the interested reader.

Question 2 also has an affirmative answer if we drop the assumption that the travel did not include stopping or backing up, but according to Boas the proof is much more difficult (it comes from a generalization of the Universal Chord Theorem proven by Hopf, that any continuous curve that has a chord of length $1$ has a parallel chord of length $\frac{1}{n}$ when $n$ is an integer.; he cites Hopf's Über die Sehnen ebener Kontinuen und die Schleifen geschlossener Wege. Comment. Math. Helv. 9 (1937), 303-319.

Answer 7

The way you argue with these people is with pretty pictures.

Answer 8

You can prove a stronger result: Either the first or the second mile of the race was run in under four minutes.

Let $t_1, t_2$ be the times for the first and second half of the race. Proof by contradiction: Suppose $t_1 > 4$ and $t_2 > 4$. Then $t_1 + t_2 > 8$.

Answer 9

Just to throw a relevant keyword into the discussion, the proof follows directly from the generalisation of the pigeonhole lemma to continuous intervals (notice that the “generalisation” section on Wikipedia describes a different generalisation which is non-continuous):

Given an interval $a$ which we decompose into $n$ parts of sizes $x_1 … x_n$ with $\sum_{i=1}^n x_i=a$ then there must be at least one $x_i$ such that $x_i \geq \frac a n$.

(I believe the generalisation is due to Gene Myers but it’s pretty easy to prove anyway.)

Adapted to your example, then $a = 7{:}59\mathrm{\,min}$, $n = 2$ and one mile must be run in ${} \geq \frac a 2$. Conversely, the other mile must then be run in $\leq a - \frac a 2 = \frac a 2 < 4{:}00\mathrm{\,min}$. $\square$

Answer 10

The argument can be explained geometrically.

Assume the time taken for each of the two miles is $x$ and $y$.

And the total time taken is: $x+y=7'59''00$ (1).

To fulfill (1) we have to meet the boundary shown in blue in Figure A.

This boundary is met if and only if $x \geq 3'59''30$ and $y \geq 3'59''30$.

Figure A: https://i.stack.imgur.com/BvIeT.png

Answer 11

Suppose to the contrary that both miles were run at a time greater than or equal to 4:00. Then put together, the two miles were covered in at least 8:00. We arrive at a contradiction. Thus our initial assumption was incorrect. One mile must have been run in less than 4:00.