In assembling a Bluetooth device, a factory uses one of two kinds of modules. One module costs $10 and the other one, that is cheaper, costs $2.5. The factory holds a $62.50 worth stock of 22 modules. How many of the modules in the stock are of the cheaper kind?	Choose the correct choice from the following answers
A) 31  B) 35  C) 21  D) 40  E) 45

Rationale: "so the number of $ 2.50 modules must be 21 so that the leftover 1 modules are of $ 10 which will give a total value $ 62.50 . 21 * 2.50 + 1 * 10 = 52.50 + 10 = 62.50 answer : c"
Answer: C