Which number can you use as a denominator to write equivalent fractions for 4/7 and 5/6

Dylan and dusty plan to take weekly surfing lessons together. if the 2-hour lessons are $20 per person and they plan to spend $100 each on new surfboards, what is the maximum number of lessons the two can take if the total amount spent for lessons and surfboards is at most $480?

Can anyone me with 32,33,36,37,and 40.

The anwser cause i do not understand just plez