It takes Avery 3 hours to build a brick wall while Tom can do it in 2 hours. If the two start working together and after an hour Avery leaves, how much time will it take Tom to complete the wall on his own?
Choose the correct answer:
A) 20  B) 30  C) 40  D) 50  E) 60
Avery takes 3 hours   Tom takes 2 hours   Efficiency of Avery is 1/3 units/hr   Efficiency of Tom is 1/2 units/hr   Combined efficiency of Tom and Avery is 1/3 +1/2 = 5/6 units/hr   Since they worked for 1 hour they completed 5/6 units of work and 1/6 units of work is left which is to be completed by Tom ( Since Avery left )   So Time taken by Tom to complete the remaining work will be 1/6/1/2 hours =&gt; 1/3*60= 
<gadget id="calculator">1/3*60</gadget>
<output>20</output>
20 minutes...   20nswer will be (20)
<result>A</result>
It takes Avery 3 hours to build a brick wall while Tom can do it in 2 hours. If the two start working together and after an hour Avery leaves, how much time will it take Tom to complete the wall on his own?