Homework help please

[drdre2001mm]

Missed class on Wednesday due to a test tonight. Anyone know how to do this problem.

A radio-active waste vessel holding the isotope Iodine-131 (I131) has been found to contain 3 x 10 raised to the 9 th nuclei. It is known that this vessel has been stored for 48 days prior to the given count. The half life of I131 is 8 days. Calculate the initial number of radioactive I131 nuclei placed in the vessel for storage. Show your work clearly in steps showing us how you thought through the problem. (Hint: Figure out how many half lives have elapsed first)