If 500g of a radioactive substance are present initially and 4 years later only 200 g remain, how much of the substance will be present after 6 years.

Question

I don't know how to get to the answer on this one.

Answer 1

A_t =A₀ (r^t) where r is the decay rate, A₀ is the initial amount, A_t is amount remaining after t years

200 = 500(r⁴⁾

r⁴ = 200/500 = 0.4 r = ⁴√ 0.4 = 0.79527

After 6 yrs, A₆ = 500(.79527)⁶ = 500(0.25298) = 126.49 gms (same end result as Dayv O's)

Answer 2

2/5=b⁴/b⁰

since 200=ab⁴

and 500=ab⁰

equation is A=500*(2/5)^t/4

check, t=4, A=200

at t=6 A=500*(2/5)^(3/2)