Be analytical.
Difference of Rs 80 in Selling Price represents 10% of Cost Price.
=> Cost Price = 80/10 x 100 = Rs 800.
Check :
Case 1 :
Profit = SP - CP = 920 - 800 = 120
Case 2 :
Profit = SP - CP = 1000 - 800 = 200
Difference in profits = Rs 80
;-)