The background radiation in a lab is 13 counts per minute.the count rate from a radioisotope is measured and i?

The background radiation in a lab is 13 counts per minute.the count rate from a radioisotope is measured and it has a reading of 119 counts per minute.if the half life of the radioisotope is 10 minutes,what will be the reading 20 minutes later?pls i need the answer urgently. Thanks