Hi,

This can be solved by finding the LCM of the two time taken by the two machines to complete the work.

The LCM of 12 & 18= 36
------------Time taken------ Units of work done per hour
Machine 1 --12 hrs-------36/12=3 units per hour
Machine 2 --18 hrs-------36/18= 2 units per hour

Now, as we know the machines do 3 and 2 units per hour, together they both will do 3+2 units per hour

Thus, the machines will together require 36/5=7 hours and 12 mins

IMO A

Hope this helps!