To make this easier (for me), I will set these fractions up as a decimal.
To do that, I will divide 100 (100% or whole) by the denominator (bottom number of a fraction).
100 / 2 = 50
Now for each 1/2 of a fraction, we get 0.50 as a decimal.
4 1/2 = 4.50
7 1/2 = 7.50
Now that we've done that, let's re-read the question.
Juliana jogs at the speed of one lap every 4.5 minutes. How long will it take for her to complete 7.5 laps?
Let's translate the question into parenthesis.
One lap every (1x) 4.5 minutes (4.5).
1 lap = 4.5m
How many minutes to complete 7.5 laps?
7.5 laps = x
Since we have 1 lap every 4.5 minutes, we need to multiply 4.5 by 7.
I only say 7 because we have 1 whole lap every 4.5 minutes, not 1/2.
7 x 4.5 = 31.5
We have 31.5 from 7 laps.
To determine how long it takes for 1/2 lap, we need to divide our time by 2.
4.50 / 2 = 2.25
It takes 2.25 minutes for 0.5 lap.
Sum up our quotient and product.
31.5 + 2.25 = 33.75.
It will take 33 3/4 (or 33.75) minutes to complete 7.5 laps.
I hope this helps!