A can do a work in 20 days and B can do it in 14 days. If they work together for 7 days, then the fraction of the work left is:

LCM of 20 and 14 is 140 which can be assumed as the total work.

A can do a work in 20 days and B can do it in 14 days.

Efficiency of A = $$\frac{140}{20}$$ = 7 units/day

Efficiency of B = $$\frac{140}{14}$$ = 10 units/day

If they work together for 7 days.

Work done by them in 7 days = $$\left(7+10\right)\times\ 7$$

= $$17\times\ 7$$

= 119

Remaining work = 140-119

= 21

Fraction of the work left = $$\frac{21}{140}$$

= $$\frac{3}{20}$$