Please help me with the wording of a question
I need to prepare a number of questions for my homework on numbers and fractions. Not being a native English speaker, I'm stuck on constructing a question.
Everyday a student solves questions in the following manner:
1st day: n questions
2nd day: n + questions=k questions
3rd day: k + questions=m questions
4th day: m + questions
(This is the part I'm stuck)
If on the 4th day she solves 648 questions, how many questions did she solve on the 1st day?
(The answer is 375.)
The question shouldn't involve any unknowns, only the numbers and 648.