Question 1124884
f(x)=7(1.15)^t
28=7(1.15)^t
1.15^t=4
t ln 1.15=ln 4
t=ln 4/ln 1.15=9,92 days

143=7(1.15)^t
1.15^t=20.43
round at end 
ln both sides
t ln 1.15=ln 20.43
t=21.59 days