What portion of the books can Pronto print in one hour?.
What portion can Red Dot do in one hour?.
So, we have
Solve for t.
1/4.5 + 1/5.5 = 1/t
(This is the same as above but a little bit more intuitive)
Solve for t:
t/4.5 + t/5.5 = 1
t/4.5 (5.5/5.5) + t/5.5(4.5/4.5) = 1
5.5t/24.75 + 4.5t/24.75 = 1
10t/24.75 = 1
(10t/24.75)(24.75) = 1(24.75)
10t = 24.75
t = 24.75/10
t = 2.475 hours