The rate of decay of the mold population is 25 mold spores per day or 25%.
The initial mold population is assumed to be 1640 spores.
The formula for exponential decay is;

Where the variables are;

We can substitute into the formula and we'll have;

For the population of spores to be 350 mold spores, the equation would now become;

At this point we shall apply the exponent rule;

Hence;

Next we apply the log rule;

We now re-write our equation as;

By use of a calculator, the value of x now becomes;

That means it would take approximately 5 days for the mold population to reach 350 spores.
Hence, for the population to be less than 350 would take a