Iodine-123, Which Is Used for Diagnostic Imaging in the Thyroid

Iodine-123, which is used for diagnostic imaging in the thyroid, has a half-life of 13 hours. If 50.0 mg of I-123 were prepared at 8:00 A.M. on Monday, how many mg remain at 10:00 A.M. on the following day?

A) 3.13 mg
B) 6.25 mg
C) 25.0 mg
D) 12.5 mg
E) 50.0 mg