Solution:
Since, there are 6 times as many goldfish as guppies then,
1 guppy → 6 goldfish
2 guppies → 12 goldfish
3 guppies → 18 goldfish
Now 3+18 = 21, the total fish in the tank
Hence there are 3 guppies and 18 goldfish
and 18-3 = 15, thus there are 15 more goldfish than guppies