is used in many home smoke alarms. if 85% of the americium in a smoke detector decays in 1250 years, what is the half-life of this isotope?

Johnny rode his bike to a friends house 4 blocks down the street in his neighborhood. he immediately rode back home once he realized his friend was unable to play. what was his displacement for the total bike ride trip? what could you use as a reference point ? show the steps to solve this problem.