3 men, 4 women and 6 children can complete a work in 7 days. A woman does double the work a man does and a child does half the work a man does. How many women alone can complete this work in 7 days

Let 1 womans 1 day work = x. Then, 1 mans 1 day work = x/2 and 1 childs 1 day work x/4. So, (3x/2 + 4x + + 6x/4) = 1/7 28x/4 = 1/7 => x = 1/49 1 woman alone can complete the work in 49 days. So, to complete the work in 7 days, number of women required = 49/7 = 7.