Sean compared to the value of the digit in the ten thousands place for the value of the digit in the tens place in this number 252,353​

cross multiplying, we get :

⇒ (3) × (5) = (9) × (x)

⇒ 15 = 9x

49/6 is going to equal 8 1/6

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: