equation csn be represented as
money left = 500 - (n-1)×4.65
so we can see in first week money left
= 500 - (1-1)×4.65= 500-0=500
which is true because first week chaya got gift card and does not spend any money.
now lets assume that on week n money left is 327.95
so by equation
500 -(n-1)×4.65 = 327.95
500- 327.95 =( n-1)× 4.65
n -1 = 172.05/4.65=37
n = 37+1 = 38
in 38th week money left would be 327.95