Idon't understand this ; -;
sally can paint a room in 7 hours and john can paint the same room in 10 hours. how long should it take sally and john to paint the room together?

The investigating team suspected that there were differences in the cost of repairing cars in workshop i and workshop ii. the investigating team suspected that the costs raised by workshop i were greater than workshop ii. for that they tested the repair of 15 cars in each workshop to see the cost of repairs.the decision of the right hypothesis to prove the suspicion above is a. h0 : μ1- μ2­ = 0; ha : μ1- μ2­ ≠ 0 b. h0 : μ1- μ2 ≥ 0; ha : μ1- μ2 < 0 c. h0 : μd ≥ 0; ha : μd < 0 with μd = μ2- μ1 d. h0 : μd = 0; ha : μd ≠ 0 with μd = μ2- μ1 e. h0 : μ1- μ2 = 0; ha : μ1- μ2 ≥ 0