Let, Total Work Unit = 120 Units
Mohan can finish in 12 days = 120 unit work
i.e. Mohan can finish in 1 days = 10 unit work
i.e. Sohan can finish in 1 days = 10+(60/100)*10 = 16 unit work
Days in which Sohan will complete the work alone = 120/16 = 30/4 = 7.5 days