Five friends went out to lunch. They each started with the same amount of money,
and they each spent $3 on lunch. They
ended with a combined total of $45. How
much did each of them have at the start?

Here's link to the

cutt.ly/tz3yDpb

don't report but were is the digram

step-by-step explanation:

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: