In this problem, we have an exponential decay function of the form

where
b is less than 1
b<1
we have
a=980 spores (initial value)
so

we know that
the population of mold decays at a rate of 20 mold spores per day
so
x=1 -----> y=980-20=960
x=2 -----> y=960-20=940
For x=1, y=960
substitute in the equation

Solve for b
b=960/980
simplify
b=48/49
therefore
the equation is

For y=250 mold spores
substitute

Solve for x

Apply log both sides

x=66 days
therefore
The number of days must be greater than 66 days
so
The answer is
67 days