i cant work out a compound interest maths problem?

Question

15 000 depreciats at 20% p.a how many years until it is half of original value

Answer 1

15000*(0.8)^x = 7500

0.8^x = 0.5

x = Log(base 0.8) 0.5

Answer 2

You are being asked to find n where
0.5 = (1-0.05)^n
The original amount of 15000 is irrelevant, since any sum would halve under the same conditions.
If the time is n years, then:
n log 0.95 = log(0.5)
n = log(0.5)/log(0.95)
= 13.513yr

Answer 3

P1 = P0 - 0.2P0 = P0(1-0.2) = 0.8P0
P2 = 0.8P1 = P0*0.8^2
P3 = 0.8P2 = P0*0.8^3
Pn = P0*0.8^n

15,000*0.8^n = 7,500
0.8^n = 0.5
nLn0.8 = Ln0.5
n = Ln0.5/Ln0.8
n = 3.1 yr.