Answer: There are 15 more goldfish than guppies.
Step-by-step explanation:
Since there are 6 times as many goldfish as guppies, then. 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.