Probability of random variable with uniform distribution on an interval

Question

Let a random variable X have a uniform distribution on the interval $[0, 10]$.
Find $P(X(X + 10) > 11)$

Since X has a uniform distribution, the pdf of X is $$ f(x)=\left\{\begin{array}{ccl}c&\text{if}&a\lt x\lt b\\ 0&\text{elsewhere}\end{array}\right. $$
Where $$c=\frac{1}{b-a} = \frac{1}{10}$$ And $P(X(X + 10) > 11) = P(X^2 + 10X - 11 > 0)$

How do I go further?

Answer 1

I've solved this without pen and paper so I'm not writing it formally but here's the solution: $X^2 + 10X - 11 > 0$ holds true if $X>1$ or $X<-11$.

In the given range that means $10 > X > 1$ which means for 90% of the total possible values of $X$.

As this is a uniform distribution all values of $X$ are equally likely, so there is a 90% probability if you do a random experiment the value of $X$ would be in $(1, 10)$.

So the probability is $0.9$.

Answer 2

If $X(X+10)>11$, then $X>1$. So the probability is 0.9. No more calculations needed.