Mr. Rafim can complete a job in 15 days, while Mr. Shafin takes twice as long to do the same. If the work together how many days will they require to complete the job?

Mr. Rafim can do 1 days=1/15 parts

and Mr. Shafin can do 1 days=1/30 parts

(Mr. Rafim+ Mr. Shafin) can do 1 days=(1/15 +1/30) parts

“         “                   “                   “              = 1/10 parts

(Mr. Rafim+ Mr. Shafin),

  1/10 parts can do = 1 day

 So,  1  part   “      “  = 10 days