A is 2 times as fast as B. A alone can do the work in 20 days. If A and B working together in how many days will the work be completed?
Choose the correct choice from the following choices
A) 23 B) 13 C) 21 D) 24 E) 25
A can finish 1 work in 20 days   13 can finish 1/2 work in 20 days - since A is 2 faster than 13   This means   13 can finish 1 work in 20*2= 
<gadget id="calculator">20*2</gadget>
<output>40</output>
40 days   Now using the awesome GMAT formula when two machines work together they can finish the job in= A13/ (A+13) = 20*40/(20+40) = 13 days   So answer is 13
<result>B</result>
A is 2 times as fast as B. A alone can do the work in 20 days. If A and B working together in how many days will the work be completed?