Step-by-step explanation:
1) given
2) 2 non - common sides in 1 ray
3) supplemental
5) opposite angles are congruent
If the two sides with length 3 and 4 are the two legs, then the missing side, i.e. the hypotenuse, is indeed 5.
But it could also be the case that 4 is the hypotenuse and 3 is one of the legs. In this case, the missing side is the other leg, so we calculate it using

So, a right triangle with legs
and 3 has an hypotenuse of 4.
Let m=n-4
n - 4 + 5n = 20
6n = 24
> n = 4
m = 4 - 4
> m = 0
(0,4)
Total people in the cafeteria = 7 + 48 + 45 = 100
There are 45 boys.
The probability would be the number of boys over the total number of people:
45/100, which simplifies to 9/20