last year, sales at a book store increased from 5,000 to 10,000. This year, sales decreased to 5,000 from 10,000. what percentage did sales increase year? what percentage sales decrease this year?

Increases and decreases are calculated by the change, divided by the old value.

For increase from 5000 to 10000, change is 5000, old value is 5000, so the increase is 5000/5000=100%.

For decrease from 10000 to 5000, change is - 5000, old value is 10000, so the decrease is - 5000/10000=-50%, but cite as 50% decrease, or increase of - 50%.