Question

It takes Jessica 1 hour 45 minutes to mow and trim the lawn.  It takes Felecia and Jessica 1 hour to do the lawn.  How long will it take Felecia on her own?

Answer 1

This kind of problem request the use of harmonic mean:

if t₁ is the Time spent by Jessica, and
t₂, the time spent by Felecia, and t₃ the time spent by both, then the equation of that kind of problem will be:

1/t₁ + 1/t₂ =1/t₃.

t₁ = 1h45 min, t₂ (unknown, to be calculated), t₃ = 1 h, to ease things let' convert the time in minutes

t₁ = 105 min

t₂ = ?(time needed to Felicia to  maw the lawn)

t₃ = 60 min

1/105 +1/t₂ = 1/60===> 1/t₂ = 1/60 - 1/105==> 1/t₂ =1/140

& t₂ =140 minutes or t₂ = 2h & 20 min