a wound, initially with an area of 80 cm^2, heals according to the formula a(t) = 80(10^-0.023t), where a(t) is the area of the wound in square centimetres after t days of healing. in how many days will 75% of the wound be healed?

When we have a 75% of the wound healed, we have the following area:
area of the wound=(100%-75%) (initial area) =25% (initial area)
In this case:
area of the wound=25%(80 cm²)=20 cm²

A(t)=80(10^-0.023t)
If A(t)=20
Then:
20=80(10^-0.023t)
10^-0.023t=20/80
10^-0.023t=0.25
log (10^-0.023t)=log 0.25
-0.023t log 10=log0.25          log 10=1
-0.023t=log0.25
t=log 0.25 /-0.023
t=26.176≈26

The wound be healed (75%) in 26 days