Explanation:
1. if the number of pages in the 1st day is 'x', then the 2d day - 'x+10', the 3d day - 'x+20', the 4th day - 'x+30', the 5th day - 'x+40' and the last day - 'x+50' pages;
2. if the sum of all the pages is 300, then it is possible to make up the equation:
x+x+10+x+20+x+30+x+40+x+50=300;
3. x=25, it means:
1st day - 25;
2d day - 35;
3d day - 45;
4th day - 55;
5th day - 65;
6th day - 75 pages.