Answer:
6 years old
Step-by-step explanation:
Given
[tex]f(t) = 200(1 -0.956e^{-0.18t})[/tex]
Required
Find t when f(t) = 140cm
Substitute f(t) = 140cm
[tex]140 = 200(1 -0.956e^{-0.18t})[/tex]
Divide both sides by 200
[tex]0.7 = 1 -0.956e^{-0.18t}[/tex]
Collect like terms
[tex]0.956e^{-0.18t} = 1 -0.7[/tex]
[tex]0.956e^{-0.18t} = 0.3[/tex]
Solve for t
[tex]e^{-0.18t} = 0.3/0.956[/tex]
[tex]e^{-0.18t} = 0.31381[/tex]
Take natural logarithm of both sides
[tex]ln(e^{-0.18t}) = ln(0.31381)[/tex]
[tex]-0.18tln(e) = ln(0.31381)[/tex]
[tex]-0.18t*1 = ln(0.31381)[/tex]
[tex]-0.18t = -1.159[/tex]
Solve for t
[tex]t = (-1.159)/(-0.18)[/tex]
[tex]t = 6.43[/tex]
[tex]t \approx 6[/tex]
