Divide 33 photos into two groups so the ratio is 4 to 7

Let x be the number of photos  in the 1st group; the number of photos in the 2nd group will be  2nd group (33 -  x).

The ratio of the 1st group to the 2nd is 4/7 OR:

x/(33-x) = 4/7 . (cross multiplication):

7x = 4(33 - x) → 7x = 132 - 4x 
 
7x + 4x = 132 → 11x = 132 and x = 12

So if x = 12 , then the 2nd group = 33-12 = 21. 

Proof: 12/21 (simplify by 3) = 4/7