Question 1095972
A(t)=10(0.8)^t; A(t) must be at least 2.
2=10(0.8)^t
2/10=0.8^t
0.2=0.8^t
take ln both sides
ln (0.2)=t ln (0.8) by property of logs
t=ln(0.2)/ln(0.8)
t=7.21 hours