Think of the amount 3/5 of an hour.
What do you multiply 3/5 hour by to get 1 hour?
You multiply 3/5 by its reciprocal, 5/3, to get 1, since 3/5 * 5/3 = 15/15 = 1.
Since you multiply the time by 5/3 to get 1 hour, you also multiply the fraction of the allowance by 5/3.
3/4 of the allowance * 5/3 = 15/12 of the allowance = 5/4 of the allowance.
---------------------------------------------------------------------------------------------------
Here is another way of thinking of this.
Turn this into a unit rate problem.
3/4 allowance for 3/5 of an hour is what unit rate of allowance per hour?
Divide 3/4 allowance per 3/5 hour.
The unit rate is 5/4 of the allowance per hour.