This is because her rate is 15 flowers per 5 minutes, and there is 45 minutes. Therefore, 5 minutes occurs 9 times in 45 minutes and fifteen flowers times 9 groups of five minutes is 135.
Because if it took 5 mins to plant 15 flowers, you can count how many 5's there are in 45 which there is 9 fives In 45 so, 15x9=135 and therefore she planted 135 flowers in 45 minutes :)
