Howmany days would p before a 2g. sample of cesium isotope had decayed so that .25 g.remained (t1/2=30 days

Halflife is 30 days, right? So after 30 days, you'd have 1g left. After another 30 days that would be 0.5g . And after another 30 days, it would turn into 0.25g. Total of 90 days, just about three months.