A population shows exponential growth of the form \( N_t = N_0 e^{rt} \) where \( N_t \) is the population at time \( t \), \( N_0 \) is the initial population size, and \( r \) is the rate of increase. If \( r = 0.1 \), then the doubling time for this population is \(\underline{\hspace{1cm}}\) (Round off to two decimal places.)