Answer:
388 (approximately)
Explanation:
So basically look at this as a fraction
if the ratio is 4 to 5 you could set the fraction as 4/5

So to get the answer, find out what you multiply to 4 by to get 310, to do this, divide 310 by 4.
310/4 = 77.5
Now multiply 5 by 77.5 to get the amount of boys.
You get 387.5, round that up to 388, (since you can't have a fraction of a person).