logarithms and exponential problem solving?

Question

Radioactive decay can be described by the following equation: 
lnA = lnAo - kt

where Ao is the original amount of the substance, A  is the amount of the substance remaining after time t , and k  is a constant that is characteristic of the substance
For the radioactive isotope strontium-80, k  is 4.08X10^-1 hours-1 ....(hours -1 means inverse im assuming?)

If the original amount of strontium-80 in a sample is 46.3 mg, how much time (in hours) is needed for the amount of strontium-80 that remains to fall to 18.8 mg?

Norebal · Accepted Answer

Ln ( 18.8 ) = Ln (46.3 ) - 4.08 x 10^ -1 ( t ) ................ 10^ -1 = 1 /10

2.93386 = 3.83514 - 4.08 t / 10 ................. - 3.83514

- 0.90128 = - 4.08 t /10 ............................. multiply by 10 and divide by - 4.08

............. t = 2.21 hrs

THE END

logarithms and exponential problem solving?

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