How to ask for the compensation of a new job offer?

I just got an offer for the part time job and a full time offer upon my graduation. Today, the HR sent me the letter only saying the pay and all of the info needed for the part time. The letter did mention that they would expect to offer me the fulltime position later on, but they never mention about the salary. The HR briefly told me about the benefits such as 401K, health insurance and PTO on the phone when informing me that i got hired.

How to appropriately ask for the pay though? Would you think this letter is just for the part time and there will be another letter for the full time?