A machine, working at a constant rate, manufactures 20 candles in 40 minutes. How many candles does it make in 1 hr 20 min? Pick one
A) 25  B) 30  C) 40  D) 45  E) 50
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= 
<gadget id="calculator">1*40</gadget>
<output>40</output>
40   The machine would be 40 candles in 1 hr 20 min.   Answer: 40.
<result>C</result>
A machine, working at a constant rate, manufactures 20 candles in 40 minutes. How many candles does it make in 1 hr 20 min?