HOW TO CALCULATE “PERCENT LESS” OF A QUANTITY?

Question

Please,How do I calculate “percent less” of a quantity.

[Let the quantity be x]

I had thought it's something like: “x – 20%”

On some answer to a question on here however,I saw: “0.8x”

It's really perplexing to me.

Or is it because the question was would lead to a fraction:

”Next-door neighbors Bob and Jim use hoses from both houses to fill Bob's swimming pool.

They know that it takes 18 h using both hoses. They also know that Bob's hose, used alone,takes 20% less time than Jim 's hose alone. How much time is required to fill the pool by each hose alone?”

In the answers

I saw “let 18/0.8x be Bob's time,a it's 20% less than Jim's time”

But I had thought it'd be ”18/x – 20%” I just don't understand.

Answer 1

20% less mean x-20%x = x-.2x = .8x

1/B + 1/J = 1/18

1/.8J +1/J = 1/18

10/8J +1/J = 1/18

5/4J + 1/J = 1/18

multiply by 36J

45+ 36= 2J

J = 81/2 = 40.5 hours alone for Jim's hose

B=.8J = (4/5)40.5 = 4(8.1) = 32.4 hours for Bob's hose alone