Question 1088710
 It takes Nicolai one and a half hours to paint the walls of a room and two hours to paint the ceiling.
 3.5 * 60 = 210 min for N to do the job
 Elena needs exactly one hour to paint the walls of the same room and one hour to paint the ceiling.
2 * 60 = 120 min for E to do the job
 If Nicolai and Elena work together, what is the shortest possible time in minutes in which they can paint the walls and the ceiling of that room?
let t = time required when working together
let the completed job = 1
A shared work equation
{{{t/210}}} + {{{t/120}}} = 1 
each will do a fraction of the job, the 2 fractions add up to 1
multiply the equation by least common denominator, 840
840*{{{t/210}}} + 840*{{{t/120}}} = 840
cancel the denominators
4t + 7t = 840
11t = 840
t = 840/11
t = 76.4 ~ 76 hrs working together, I think you are right