the ratio of dogs to cats in the neighborhood was 3 to 7. If a total of 210 dogs and cats were in the neighborhood, how many cats were there?
or,
3x+7x = 210
10x = 210
x = 21
dogs = 3x = 63
cats = 7x = 147
The above answer is clearly wrong, since the total animals is only 210, not just the dogs.
d/c = 3/7
7d = 3c
d+c = 210
3d+3c = 630
3d+7d = 630
10d = 630
d = 63
so, c=147