You've brought 5 cakes to the school dance and you want to be sure there's
enough cake for everyone.
Here's a formula for this scenario:

y=xa/5

X = number of people at the dance
y = number of slices per cake
a = number of slices each person will eat

b = (X*a)/y whereas b is the number of cakes.

Step-by-step explanation:

answer: 45

a 4/6 = 9/12   not proportional

step-by-step explanation:

to find which are not proportional, we can use cross products

if it is true, they are proportional.   if it is not true, they are not proportional.

a 4/6 = 9/12

4* 12 = 6*9

48 = 54

not this is not proportional  

b 2/3 = 12/18  

2* 18 = 3 * 12

36 = 36

this is proportional

c 5/4 = 10/8  

5*8 = 4*10

40 = 40

this is proportional

d 7/5 = 35/25

7*25 = 5 * 35

175 = 175

is proportional