Question

A can do a piece of work in 14 days ,B can do it in 21 days . They began together and worked at it for 7 days . Then ,A was not well and B had to complete the work . In how many days was the work completed ?

Answer 1

A can do a piece of work in one day = 1/14

B can do a piece of work in one day =1/21

both can do in one day

$\frac{1}{14} + \frac{1}{21} \\ \frac{3 + 2}{42} \\ \frac{5}{42}$

in seven days they can do

$\frac{5}{42} \times 7 \\ \frac{5}{6}$

reaming work is

$1- \frac{5}{6} \\ \frac{6 - 5}{6} \\ \frac{1}{6}$

B can do a work in 21 days and 1/6

then

$21 \times \frac{1}{6} \\ \frac{21}{6} \\ 3.5$

hence the B can complete the remaining work in 3.5 days.