Amoeba-doubling problem, help please

Hi everyone, I'm new to the forum. I'm taking an education course for math teachers. I got the following homework problem wrong, and the teacher won't tell me why. Please help. BTW, the teacher is awful, and everyone in the class got the same wrong-answer I did.

Verbatim: An amoeba is placed in a jar at 1PM. It reproduces by doubling every twenty minutes. How many amoebas will be in the jar just after 5PM that day?

2^12=4096 isn't the answer, I'm told. Thanks for your help.