When positive integer N is divided by positive integer P, the quotient is 18, with a remainder of 7. When N is divided by (P + 2), the quotient is 15 and the remainder is 1. What is the value of N?  Pick
A) 151
B) 331
C) 511
D) 691
E) 871

Rationale: "use the rule dividend = ( integer quotient ) * ( divisor ) + remainder to translate each sentence . the first sentence becomes n = 18 p + 7 . the second equation becomes n = ( p + 2 ) * 15 + 1 , which simplifies to n = 15 p + 31 . these are ordinary simultaneous equations since they both equal n already , let ’ s set them equal and solve for p . 18 p + 7 = 15 p + 31 3 p + 7 = 31 3 p = 24 p = 8 now that we know p = 8 , we can just plug in . the product 15 * 8 is particularly easy to do , without a calculator , by using the “ doubling and halving ” trick . double 15 to get 30 , and take half of 8 to get 4 — - 15 * 8 = 30 * 4 = 120 . so n = 15 ( 8 ) + 31 = 120 + 31 = 151 n = 151 , answer = a ."
Answer: A