Sandy gets 3 marks for each correct sum and loses 2 marks for each incorrect sum. Sandy attempts 30 sums and obtains 55 marks. How many sums did Sandy get correct?
Choose the correct choice:  A) 19  B) 21  C) 23  D) 25  E) 27

Rationale: "let x be the correct sums and ( 30 - x ) be the incorrect sums . 3 x - 2 ( 30 - x ) = 55 5 x = 115 x = 23 the answer is c ."
Answer: C