50 yuan has: (410-20x10)/(50-20) = 7 (sheets).
20 yuan has: 10-7=3.
Analysis: Assume that 10 is from 20 yuan, and the total number is 20x10 = 200.
Actually, 4 10 is 4 10-200=2 10, which is more than 200 yuan.
And more than 50 to 20 is 50-20 = 30 yuan.
So the number of 50 sheets is 2 10/30=7 (sheets).