Question

If 1.7 of a substance decomposes in 50 years, what percentage of the substance will remain after 100 years? how many years will be required for 10% to decompose?

Answer 1

$A= Pe^{rt}$ is your formula
0.17=1.7e^{0.1t}       10% of 1.7 is 1.7*0.1=0.17
0.1=e^{0.1t}
ln(0.1)=0.1t
t=  ~23 years

if I assume the values used are correct, it will take approximately 23 years for 10% to decompose.