If a radioactive sample contained 1600 billion nuclei and its half life was 57 years, after how many years would there be 400 billion nuclei remaining?

Ineed the answer to question 1 , i’m desperate

Ametal ring 4.20 cm in diameter is placed between the north and south poles of large magnets with the plane of its area perpendicular to the magnetic field. these magnets produce an initial uniform field of 1.12 t between them but are gradually pulled apart, causing this field to remain uniform but decrease steadily at 0.240 t/s . (a) what is the magnitude of the electric field induced in the ring? (b) in which direction (clockwise or counterclockwise) does the current flow as viewed by someone on the south pole of the magnet?

What describes the formation of horizon b? a. forms at the surface b. features parent material c. undergoes the most change d. forms due to decomposed material i think the answer is c. undergoes the most change