Answer:
(D) 2/3
Explanation:
together they will be faster than the fastest single time (1 hour).
so, we need units of time that are smaller than 1 hour to work with both workforces.
so, let's use 1 minute as a basic unit.
Mary can do in 1 minute 1/60 of the work.
Simon can do in 1 minute 1/120 of the work.
together they do in 1 minute
1/60 + 1/120 = 2/120 + 1/120 = 3/120 of the work.
out of the 2 + 1 = 3 units of work done, she will have done 2 units, and he 1 unit.
so, the fraction of her work is for every minute (until the work is done) 2/3.
therefore, when the work is done, the same fraction applies. 2/3 if the whole wall will haven been painted by Mary.
FYI : together it will take them to paint the whole wall (the whole is 1 or 120/120) :
3/120 × x = 1
x = 120/3 = 40 minutes