How to solve this using equations?

Question

if there are 353 million fish, cats, and dogs, and there are 7 million more fish than cats and dogs combined, and 15 million more cats than dogs, how many are there of each?

Answer 1

f = fish 

c = cats 

d = dogs 

 

1. f + c +d = 353 m

 

2. f = c + d + 7 m 

 

3. c = d + 15 m 

 

from 2 and 3 

 4. f  =  2d + 22 m 

 

sub f from 4 and c from 3 in 1 to get d 

 

2d + 22 + d + 15 + d = 353 

 

4d  = 353 - 37  = 316 

 

d = 316 / 4 = 79 

 

c = 79 + 15 = 94 

 

f = 353 - 94 - 79 = 177