2 answers
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50 = 40(1.08)^n
50/40 = 1.08^n
n log 1.08 = log(5/4)
n = log(5/4) / log 1.08
= 2.899 or appr 2.9 years
check:
40(1.08)^2.9 = 50.002
Your expression is incorrect