Since the yeast divides every two hours, its population (and hence yeast mass) doubles every two hours. Because the doubling time is a constant it is exponential growth.

The mass of yeast grows exponentially with time as : #m(t)=m_o\exp(\lambdat); \qquad => \lambda = \ln(2)/\tau#,
where #\lambda# is the growth factor and #\tau# is the doubling time.
We are given the doubling time (#\tau#) and the initial mass #m_0# and are asked to estimate #t# for #m(t)# to reach the value of the mass of a typical human. Let us assume #75# kg as the mass of a typical human, so #m(t)=75# kg.