Question 973368
 y = 20.2 log(x)
log(x) = y / 20.2
we are given y = 15
log(x) = 15 / 20.2 
0.742574257 = log(x)
now use definition of logarithm
x = 10^(0.742574257)
x = 5.528079212
the tree is approx 5.5 years old