Dean walked from the library to the mall. For the first 12 minutes, he walked at a speed of 80
m/min, but then he slowed his speed to 68
m/min for next 7 minutes. If he was still 130 m
away from the mall, what was the distance
between the library and the mall?

at least two

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: