Question

The probability, P, in percent, that a person will respond to a certain ad can be modelled by the equation P(n)=1-2^(-0.05n), where n is the number of days since the ad began on tv. After how many days is the probability greater than 20%?

my work:
0.20=1-2^(-0.05n)

then i solved for n, but i got a negative number.. what is wrong with my equation that i made and why