For one roll, you need to take that 1/3 cup and split it into 5 parts. To do this, divide by 5, which means the fraction becomes 1/15, giving you 1/15 cup flower for one roll.
36 rolls is just 36 times the one roll, which is 36/15 cups. You can divide both parts by 3 to make that 12/5 cups for 36 rolls.