The speed em werk: (1/a)+(1/b)=1/12
speed av a=1/21=0.047619047
kombined speed=1/12=0.0833333
speed av b=0.083333-0.047619047=0.0357142=1/28 daes
after 6 daes tugether, get half the werk dun, so half left
now b werk alone tu du (1/2), so it take him 14 daes more