double
from initial amont of P to final amount of 2P
so
2P=P(1.071)^time
divide both sides by P
2=1.071^time
take the ln of both sides
ln2=timeln1.071
divide both sides by ln 1.071
(ln2)/(ln1.071)=time needed
using calculator
10.1052=time needed
about 10.1 hours