I read 1/5 on the first day, which is 300* 1/5=60 pages.

Read the remaining 25% the next day, that is, (300-60)*25%=60 pages.

You can ask questions like this:

First question: How many pages did you read on the first day?

Answer: 300* 1/5=60 pages.

Second question: How many pages did you read the next day?

A: (300-60)*25%=60 pages.

You can also ask this question:

First question: How many pages have you read?

A: 60+60= 120 pages.

Second question: How many pages are left?

A: 300-60-60= 180 pages.