Ten masons can build a wall working seven hours a day, in twelve days. In how many days can the work be completed by fourteen persons working eight hours a day.

Answer : 
Wall can build by 

10 masons , working 7 hr for  number of  days   = 12

1 masons , working 1 hr for number of  days   = 12 $\times$ 10  $\times$ 7  days
So,
14 masons , working 8 hr for  number of  days   =  $\frac{12 \times 10 \times 7}{14 \times 8}$  =  7.5 
So,
14 masons , working 8 hr for  number of  days   = 7.5 days to build a wall