A x 1.1 =B

B x 0.9 = 990

so B = 990/0.9 = 1100

so A = 1100/1.1 = 1000

A is the price Ron bought the watch for, B is the price Sam bought the watch for (A + 10%, also written as A x 110% since A = 100% of A)

in the same way B x 0.9 (90%) is $990, the price paid by Mike

so Ron paid $1000 for the watch

Sam got ripped off

Mike made a good deal

you can thank me now kid