Calculating probability from cumulative frequency graph

Question

I have this graph cumulative frequency graph

and this problem related to this graph:

A student is chosen at random. The probability that the student estimates that the mass is greater than M grams is 0.3. Find the value of M.

I thought of doing it with relative frequency formula

0.3 = M/200

M = 0.3*200

But the answer i got was far from the right answer,so how do i do this?

Answer 1

Let $\hat{M}$ be the randomly chosen student's estimation. Then $\Bbb{P}(\hat{M} > M) = 0.3$, so $\Bbb{P}(\hat{M} \le M) = 0.7$. Since you have $200$ students, you need to read from the horizontal line $200 \times 0.7 = 140$ and find the corresponding value of $M$ from the curve. In this graph, it's $M = 4$.