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