1. He had to check the oven every 15 minutes.

Question

2. He had to check the oven every 15th minute.
(Are both grammatical?)

3. Mom will give me turkey sandwich for lunch for the next two week.
(Is 'for lunch' an adjective pharase or an adverbial phrase? Does 'for lunch' modify 'turkey sandwiches' or 'gave'?)

Answer 1

1. is correct. 2. is not used.

3. "For lunch" is an adverbial phrase modifying "will give."