Final answer:
It takes 8 hours for a 100g sample of an isotope with a 4-hour half-life to decay to 25g, as each half-life results in half of the remaining isotope decaying.
Step-by-step explanation:
The question requires us to determine how long it will take for a 100g sample of an isotope with a half-life of 4 hours to decay until only 25g remains. Since the half-life represents the time required for half of the isotope to decay, we can use this information to calculate the total time for the desired amount of isotope to remain.
After one half-life (4 hours), 50g of the isotope remains. After a second half-life (another 4 hours, totaling 8 hours), half of the 50g will decay, leaving 25g. Therefore, it takes 8 hours for the original 100g of isotope to decay to 25g.