First you'll need two equations ( you can use any variables you want) A= photo group 1 B= photo group 2 First equation: A + B = 33 This is because the total number of photos from both groups is 33 Second equation: 4A = 7B This is because the ratio is 4:7 so for every four photos in group A there must be 7 photons in group B. Then take the first equation A + B = 33 and subtract B from both sides. Making the equation A = 33 - B Then with that equation, substitute it in for equation 2 and it will look like this. 4(33-B) = 7B Then distribute the 4 to the 33 and the -B making the equation 132 - 4B = 7B Then add 4B to each side Making the equation 132 = 11B Then to get B by itself you must divide each side by 11 making the equation 132/11 = B which simplifies to 12 = B Then plug in 12 for B in the first equation like this A + 12 = 33 Then subtract 12 on each side of the equation to isolate A so A = 21