Question

A biochemist used 10.000 grams of P-32 in a test on plant growth but forgot to record the date of his experiment. When he measured the amount of phosphorus-32 on another day, he found only 1.250 grams of the isotope remaining. If the half-life of P-32 is fourteen days, when did he start his experiment? And could you please explain how you did it!? Thanks! :)