It takes Avery 2 hours to build a brick wall while Tom can do it in 4 hour. 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 choice from the following choices
A) 70
B) 20
C) 50
D) 60
E) 30
Avery takes 2 hours   Tom takes 4 hours   Efficiency of Avery is 1/2 units/hr   Efficiency of Tom is 1/4 units/hr   Combined efficiency of Tom and Avery is 1/2 +1/4 = 3/4 units/hr   Since they worked for 1 hour they completed 3/4 units of work and 1/4 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 1/4/1/4 hours =&gt; 1*60= 
<gadget id="calculator">1*60</gadget>
<output>60</output>
60 minutes...   Answer will be (60)
<result>D</result>
It takes Avery 2 hours to build a brick wall while Tom can do it in 4 hour. 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?