3 grade math
posted by hiba .
if a man cut grass in 6 hour , another man cut in 12 hour , how many hour ther were both spend if they cut grass toghter.
answer 9 hour or 4 hour

plz answer ....plz

rate of first man = 1/6 lawn/hr
rate of 2nd man = 1/12 lawn/hr
combined rate = 1/6 + 1/12 = 3/12 or 1/4 lawn/h
time takes when both cutting = 1/(1/4)
= 1 (4/1) = 4 hrs
(how can the combined work take 9 hours, if one guy alone can do it in 6 hrs??) 
thank you so much .........

12+6=18