Hey there!
This is an example of conditional probability, or P(A | B). Let's say that the probability of a child eating cake is event A, and the probability of them eating cookies is event B.
Conditional probability (A, given that B has occurred) can be represented and found by the following equation:
P(A | B) = P(A ∩ B)/P(B)
P(A ∩ B) is the probability of a child eating both cake and a cookie. This is also the middle of the venn diagram you were given. P(B) is simply the probability of event B happening, which, as we established, is eating a cookie.
P(A | B) = P(A ∩ B) / P(B)
P(A | B) = 0.1 / 0.5
P(A | B) = 0.2
There will be a 0.2, or 20%, chance that a child will eat cake, given that they've eaten a cookie.
Sorry for the late response, but I hope this still helped you out!