Math mystery rogue runner 6th graders

what are the answers for clue 3? i am confuse

is means division by fractions

step-by-step explanation:

okay..i will pick 8!

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:  

triangle eg 7.5 × cos "a" or 7.5 × sin "b" or 6 tan"a"