90:10 is 9:1 (90/10=9)
160:20 is 8:1 (160/20=8)
80:10 is 8:1 (80/10=8)
120:20 is 6:1 (120/20=6)
The lowest ratio is 6:1 or 120:20.
[Now it's time for bed, 25 past midnight!]
(If the ratio was reversed number of lessons to cost, the lowest would be 10:90 and 20:120 would be largest.)