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
