A can do a job in 15days and B in 20days. If they work on it together for 3 days, then the fraction of the work that is left is? Choose the correct choice:
A) 2/15
B) 8/15
C) 13/20
D) 1/12
E) 6/13
A's 1 day work = 1/15   B's 1day work = 1/20   A+B 1day work = 1/15 + 1/20  +  1= 
<gadget id="calculator"> +  1</gadget>
<output>1</output>
1/15 + 1/20= 7/60   A+B 3days work = 7/60*3 = 7/20   Remaining work = 1 - 7/20 = 13/20   Answer is 13/20
<result>C</result>
A can do a job in 15days and B in 20days. If they work on it together for 3 days, then the fraction of the work that is left is?