How many half -lives are required for three-fourths of the nuclei of an istope in a sample decay

The half life of a radio-isotope is the amount of time required for half of the isotope to decay into a new element or sister isotope. One half life results in a 50% loss of radioisotope, and two half lives results in a 75% loss. For three-fourths of the nuclei to decay, two half-lives must pass.