Percentages

Hi

How do you determine the format of using percentage in a question. For example in some questions you see where like the format used is 115/100 or 100/85 or as is as 15/100.

This question i was doing from a question bank for the F2 exam stated:
A unit of product L requires 9 active labour hours for completion. The performance standard for product L allows for ten percent of total time to be idle, due to machine downtime. The standard wage rate is $9 per hour. What is the standard labour cost per unit of L.

The answer read
9 hours * 100/90 * $9 = $90.

I find it a bit confusing at times.