Solve the problem using an equation
Pronto Press can print an order of booklets in 4.5 hours. Red Dot printers can do the same job in 5.5 hours. How long will it take if both presses are used?
What portion of the books can Pronto print in one hour?.
$\displaystyle \frac{1}{\frac{9}{2}}=\frac{2}{9}$
What portion can Red Dot do in one hour?.
$\displaystyle \frac{1}{\frac{11}{2}}=\frac{2}{11}$
So, we have $\displaystyle \frac{2}{9}+\frac{2}{11}=\frac{1}{t}$
Solve for t.
Because it allows you to set up an equation which is easier to solve.
Solve the equation I gave at the bottom of the page.
You could also consider:
$\displaystyle \frac{2t}{9}+\frac{2t}{11}=1$
Solve for t.
It is set equal to 1 because that is 1 whole job.
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