Question 984029
This is basically an interest problem with the percentage negative.
P=Po{1-0.08}^n
But P/P0=0.5, since it will be half as much.
0.5=1.08^n
take ln of both sides
0.693=n ln (1.08); ln (1.08)=0.07696 (rounded).
Not rounding, 
divide by that.
9.006 years (rounded)
This is the rule of 72 in reverse.