急 ! Logarithmic Function

27.
depreciates after n years:
450000 (1-20%)^n < 100000
0.8^n < 2/9
log (0.8^n) < log (2/9)
n log (0.8) < log (2/9)
n > log (2/9) / log (0.8) [inequality sign change because log (0.8) < 0]
n > 6.74

28.
10000 (1+1%)^n < 11000
1.01^n < 1.1
log (1.01^n) < log (1.1)
n log (1.01) < log (1.1)
n < log (1.1)/log (1.01)
n < 9.58

29.
N8 = 300 log (8)
Nt = N8 + 90 = 300 log (t)
300 log (8) + 90 = 300 log (t)
3 = 10 log (t) - 10 log (8)
3 = 10 log (t/8)
log (10^3) = log [(t/8)^10]
10^3 = (t/8)^10
10^(3/10) = t/8
t = 8 x 10^(3/10)
t = 16 (correct to nearest integer)