Question

What is the ratio?

Answer 1

is there like a paragraph before this to explain the scenario?

Answer 2

@Javk it says :

Answer 3

oh ok ... so for every 645 mutant flies there were 355 wild-type flies
meaning,
mutant flies : wild-type flies
just substitute the values now,
\[645 : 355\]
now simplify, so you can divide both sides by 5
\[129 : 71\]
71 is a prime no. so you can't factorize any farther. Meaning this is your final answer.