It takes Avery 4 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?
Select:
A) 20 B) 40 C) 30 D) 50 E) 60
Avery takes 4 hours   Tom takes 2 hours   Efficiency of Avery is 1/4 units/hr   Efficiency of Tom is 1/2 units/hr   Combined efficiency of Tom and Avery is 1/4 +1/2 = 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 1/4/1/2 hours =&gt; 1/2*60= 
<gadget id="calculator">1/2*60</gadget>
<output>30</output>
30 minutes...   Answer will be (30)
<result>C</result>
It takes Avery 4 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?