After one hour we the number of bacteria doubles (2n). After two hours the number of bacteria doubles again (2x2n=4n). After three hours the number of bacteria doubles again (2x2x2n=8n). so we have a geometric series of 2t, where t represents the number of hours from the beginning. So out equation looks like
2tn = number of bacteria at time t
224(100) = number of bacteria after 24 hours when starting with 100