Discussion :: Time and Work

A can do a work in 15 days and B in 20 days. If they work on it together for 4 days, then the fraction of the work that is left is :
A.
\( \frac { 1 } { 4 }\) 
B.
\( \frac { 1 } { 10 }\) 
C.
\( \frac {7} { 15 }\) 
D.
\( \frac {8 } { 15 }\) 
Answer : Option D
Explanation :
A's 1 day's work =\( \frac { 1 } { 15 };\)
B's 1 day's work =\( \frac { 1 } { 20 };\)
(A + B)'s 1 day's work =\( [\frac { 1 } { 15 }+\)\( \frac { 1 } { 20 }]\)=\( \frac {7} { 60 }\)
(A + B)'s 4 day's work =\([ \frac {7} { 60 }*4]\)=\( \frac {7} { 15 }\)
Therefore, Remaining work =[1\( \frac {7} { 15 }\)]=\( \frac {8 } { 15 }\)
I am prepare for job
Where from the 7/60 because I don't understand
How do they arrive to 7/60 in the first place
How do they substract 17/15
why cant we take work done by A in 4 DAYS as 4/15 and work done by B in 4 days as 4/20 and add A'S work and B'S work AND THEN SUBTRACT (4/15+4/20 = 2/5) LEFT OVER WORK =12/5=3/5