P and Q can do a work in 8 days. Q and R can do the same work in 12 days. P, Q and Rtogether can complete it in 6 days. P and R together can do it in how many days?

$$work done= efficiency \times time$$

                    P+Q                     Q+R             P+Q+R

time             8 days                  12 days           6 days

Total work = LCM(8,12,6)

                 =24

                 P+Q                      Q+R               P+Q+R

efficiency    3                           2                   4

From this 

efficiency of P=2 , R=1

$$24= 3 \times time $$

hence, time=8 days