Answer:
After 1 decay it has .9375 of its value left
.9375^n = 1/2
What is the value of n (number of decays)
n log .9375 = -.693
n =.693 / .0645 = 10.7
After 10.7 periods of 50 sec it will have 1/2 of its original nuclei left
1/2 life = 10.7 * 50 sec = 537 sec = 8.95 min