The circus sells two kinds of tickets: lower seats for $30 and upper seats for $20. On a certain night, the circus sells 80 tickets and gets $2100 in revenue from the sales. How many tickets for lower seats did they sell? Choose the correct choice.
A) 40 B) 45 C) 50 D) 55 E) 60

Rationale: let l be the number of lower seat tickets . let u be the number of upper seat tickets . l + u = 80 and u = 80 - l . 30 l + 20 u = 2100 . 30 l + 20 ( 80 - l ) = 2100 10 l + 1600 = 2100 . 10 l = 500 . l = 50 . the answer is c .
Answer: C