Explanation:

Ta= 12 hours

Tb= 15 hours 

Tc= 20 hours 

lcm ( total work ) = 60 units 

=> Ea = 60/12 = 5 units per hours 

    Eb = 60/15 = 4 units per hours 

    Ea = 60/20 = 3 units per hours 

on first hour ( A+B ) will work = 5+4 = 9 units 

on second hour ( A+C ) will work = 5+3 = 8 units 

so => 2 hour work = 17 units .

=> 6 hour work = 17 x 3 = 51 units 

remaining work = 9 units .

so next work will be done by 

(A+B) = 9 units 

so, work will be completed in 7 hours