N can do a piece of work in 80 days. He works at it for 10 days and then M alone finishes the remaining work in 42 days. In how much time will N and M, working together, finish the work?

এখানে ,N,80 দিনে করে =1 অংশ
          N,10  "        "      =10/80 =1/8  অংশ
 বাকি =1-1/8=7/8 অংশ

            M,7/8 অংশ করে 42 দিনে
অতএব,   1      "         "(42× 8)/7=48 দিনে
অতএব,M ও N একত্রে কাজটি করে (48×80)/(48+80)
                                                     =(48×80)/128
                                                     =30 দিন