Abookshop has a sale.
a book normally costing $39.95 is priced at $23.97.
by what percentage has the price been reduced?

show working out

The price been reduced by a percentage of 40%

Step-by-step explanation:

we know that

$39.95 > represent the 100%

so

The difference of prices is equal to ($39.95-$23.97)=$15.98

Using proportion

Find the percentage of the difference

100/39.95=x/15.98

x=100*15.98/39.95

x=40%

ending amount = beginning amount / (2^n)

where "n" = number of half-lives (or elapsed time / half-life)

ending amount = 50 mg / (2 ^10 / 5.3)

ending amount = 50 mg / (2 ^ 1.8867924528)

ending amount = 50 m / 3.6981210597

ending amount = 13.5203794557 mg

or about 13.5 mg remain

source: