From 1994 to 1995 the sales of a book decreased by 80%. if the sales in 1996 were the same as in 1994, by what percent did they increase from 1995 to 1996?

400%

Suppose sales in 1994 were 100 of some unit. Then in 1995, they were ...

... 100 - 80% × 100 = 100 × (1 - 0.80) = 100 × 0.20 = 20 . . . . units

Then the percent increase to sales of 100 units in 1996 can be found from ...

... percent change = ((new value) - (old value))/(old value) × 100%

... = (100 -20)/20 × 100%

... = 80/20 × 100%

... = 400%

The increase from 1995 to 1996 was 400%.