Which number is largest 7.2 x 10 to the negative 6 power, 3.09 x 10 to the 3rd power, 2.04 x 10 to the fourth power, 5 x 10 to the 3rd power

Afitness center has 2 membership plans one has a low dollar sign up fee of 15 dollars and costs 38 dollars per month to join. a second plan has a higher sign up fee of 78 dollars but a lower monthly fee of 31 dollars for what number of days of the month is the coat the same