Question

suppose 100 radioactive atoms have a half life of 10 seconds. Approximately how many radioactive atoms will be left after 20 seconds.

50
12.5
25
75

Answer 1

25 as two half lives are happening :D

1/2 * 1/2 = 1/4

1/4 * 100 = 25

Answer 2

half life means the time taken for the amount of a substance undergoing decay to decrease by half its initial value, so
in 10 seconds it decreases from 100 to 50,
in 20 it decreases from 50 to 25