If there are twenty-one in total, then how many are in one-seventh? (Hint: Divide 21 by 7.)
Then how many are in two-sevenths? (Hint: Multiply the number in one-seventh by 2.)
Then how many are left after he's had his? (Hint: Subtract the number for two-sevenths from 21.)
Given how many were left over, what is one-fifth of this? (Hint: Divide by 5.)
Then how many are three-fifths of this? (Hint: Multiply the number for one-fifth by 3.)
Then how many are left, after she's had hers? (Hint: Subtract the number for three-fifths from the number she's started with.)