Let x be the number of guppies, y the number of goldfish and z the number of zebrafish.
1) x = z + 3
2) z = 2y or y = z/2
3) x + y + z = 23
substitute eq (1) and(2) into eq(3):
(z + 3) + (z/2) + z = 23
2.5z + 3 = 23
2.5z = 20
z = 20/2.5 = 8
using eq(1):
x = z + 3 = 8 + 3 = 11
using eq(2):
y = z/2 = 8/2 = 4
check using eq(3):
11 + 4 + 8 = 23
There are 8 zebrafish, 11 guppies and 4 goldfish.