A machine, working at a constant rate, manufactures 20 candles in 40 minutes. How many candles does it make in 1 hr 20 min?	Choose the correct choice from the following choices:  A) 25  B) 30  C) 40  D) 45  E) 50

Rationale: "change 1 hr 20 min to 80 min . for this , we need to set up a simple proportion of staplers per time 20 / 40 = s / 80 the absolutely worst thing you could do at this point in the problem is to cross - multiply . that would be a supremely unstrategic move . instead , cancel before you multiply . for what we can and can ’ t cancel in a proportion , seethis post . we can cancel the factor of 10 in the 20 and 40 . 2 / 4 = s / 80 1 / 2 = s / 80 ; 1 / 2 * 2 = s / 80 * 2 1 / 1 = s / 40 now that the fraction is entirely simplified , we can cross - multiply . s = 1 * 40 = 40 the machine would be 40 candles in 1 hr 20 min . answer : c ."
Answer: C