After the first week you have a profit of 200*($37-$25) = $2400

Buying in the second week you have ($2400+$5000)/$25 = 296 units. Profit will be 296* ($37-$25) = $3552

Buying in the third week you have ($2400+$5000+$3552)/$25 = ... units. Profit will be ...* ($37-$25) = ...

And so on.