Answer: 6th week
Step-by-step explanation:the number of weeks is moving in a geometric proportion
Remember Tn = ar^n-1
Tn = 50, r = 800/1600= 1/2, a = 1600
So 50 = 1600 * 1/2^ n-1
50/1600 = 1/2^ n-1
1/32 = 1/2^ n-1
2^ -5 = 2^ -n+1
Equating powers
-5 = -n + 1
-n = -6
n = 6