a certain radioactive substance has a half-life of 40 minutes. How much of an original 100 grams substance will remain after 2 hours?

half life = 40minutes =2/3 hours

t_(1/2)=(ln2 /k)

then k=(ln2/t_(1/2)) = ln2/(2/3) = 3/2*(ln2)

origianl substance y₀= 1000 grams

y=y₀e^-kt

y= 1000*(e^{-(3/2(ln2))*2} ) = 1000/8 = 125 grams

m= m_0/2^t/T

m=100 gms..

t=2 hours=2*60=120 min..

T=40 min

m=100/2^120/40

m=100/2³

m=100/8

m=12.5 gms.....