How do i explain how to get an answer to my 6th grade granddaughters : divide 33 photos into 2 groups so the ratio is 4: 7

33 photos in a 4x to 7x ratio

4x + 7x = 33
11x = 33
x = 3

33 photos can be put in a 4(3) to 7(3) ratio

which is a 12 to 21 ratio

so 12 photos on one side, and 21 on the other side to get the ratio of 4 to 7

Note:
a(b) = ab = a x b = a times b

The correct answer is: b