a is twice as good a workman as b and they took 7 days together to do the work b alone can do it in .
"c 21 days wc = 2 : 1 2 x + x = 1 / 7 x = 1 / 21 = > 21 days"
multiply(divide(multiply(7, add(const_2, const_1)), const_2), const_2)