Ben has 3 times as many guppies as goldfish if he has a total of 20 fish how many guppies does have?

Guppies: u
Goldfish: o

3 times as many guppies as goldfish
3*o = u   (there are more guppies than goldfish)

o + u = 20
o = 20 - u

Substitute in (20 - u)  for o in the first equation and solve:
3* (20 - u) = u
60 - 3u = u
60 = 4u
u  = 15

He has 15 guppies. Then substitute in the number of guppies into either equation to find the number of goldfish:
o + u = 20
o + (15) = 20
o = 5

Hope this helps! :)