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?

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