susy: x
mary: y
x=2/3y
x+1/2y=210
therefore
2/3y+1/2y=210
4/6y+3/6y=210
7/6y=210
y=180 (Mary)
x=180*2/3=120 (Susy)
easy peazy
so write like this:
20 people -> 4800 @36min
9 people -> 3600
now we have to find efficiency of each person
20*36min*efficiency = 4800
efficiency = 6.66667
therefore, 9 people * 6.6667 * time = 3600
time = 60 minutes
Easy peazy
