Explanation:

Let efficiency of A is x, so of B = (140/100)*x = 7x/5
So ratio of efficiencies = x : 7x/5 = 5 : 7
So ratio of days = 7 : 5
A can do in 21 days, so 7y = 21, y = 3
So B can do in 5*3 = 15 days
A+B in (21*15)/(21+15) = 8 3/4 days