Two grandfathers can nit a sweater in 6 days

Two grandfathers can nit a sweater in 6 days. Two grandfathers and one grandmother can nit a sweater in 3 days. How many days will it take the grandmother to nit a sweater all by her self?

(a) 4.5.
(b) 5.
(c) 5.5.
(d) 6.
(e) 6.5.

The best answer is D.
Two grandfathers and a grandmother can nit a sweater in 3 days, therefore they can nit 2 sweaters in 6 days. Because two grandfathers can nit 1 in 6 days then the other sweater is done by the grandmother, she can nit 1 sweater in 6 days.