X, Y, Z working alone finish constructing a house in 4, 6 and 12 hrs. resp. X starts alone and works for 1 hr. He is then joined by Y and they work together for 1 hr. Finally Z joins them and they all work together to finish the work. If they start working at 9 am, at what time will they finish?
Choose the correct option
A) 11.30 am  B) 11.35 am.  C) 11.40 am.  D) 12 am  E) 11.20am
Let the total work be 12 units   Efficiency of X = 3 units/ hour   Efficiency of Y = 2 units/ hour   Efficiency of Z = 1 units/ hour   So, X completes 3 units of work by 10 am   So, X and Y completes 5 units of work by 11 am   Work left is 4 units ( 12 - 3 - 5 ) and combined efficiency of X, Y and Z is 6 units   So, time required to complete 4 units will be 4/6 =&gt; 4/6*60= 
<gadget id="calculator">4/6*60</gadget>
<output>40</output>
40 minutes...   So they will require 40 minutes after 11 am to complete the remaining work...   Hence answer will be (11.40 am.) 11:40 am
<result>C</result>
X, Y, Z working alone finish constructing a house in 4, 6 and 12 hrs. resp. X starts alone and works for 1 hr. He is then joined by Y and they work together for 1 hr. Finally Z joins them and they all work together to finish the work. If they start working at 9 am, at what time will they finish?