a is 2 times as fast as b . a alone can do the work in 20 days . if a and b work on alternative days , beginning with a , in how many days will the work be completed ?
"a can finish 1 work in 20 days b can finish 1 / 2 work in 20 days - since a is 2 faster than b this means b can finish 1 work in 20 * 2 days = 40 days now using the awesome gmat formula when two machines work together they can finish the job in = ab / ( a + b ) = 20 * 40 / ( 20 + 40 ) = 13 days since they alternate on days , it will take them double this time so total time = 13 * 2 = 26 days so answer is e"
divide(const_1, add(divide(const_1, 20), divide(divide(const_1, 20), 2)))