To find the percent of the maximum number (70) you have to multiply the decimal form of the percent by 70. Once we do this, we get these answers:
Rainbow fish: 14
Swordtail: 28
Molly: 21
In Part B, he has not filled the maximum number of fish because we can see that the percent of fish in his tank do not add up to 100%. The remaining percent that can still fit fish is 10%, and 10% of 70 is 7.