estimation is to round to easy numbers first and then multiply or divide, which can then be done easily in our heads.
so, for 6. I rounded to 60 pages per day and 30 total days (the nearest tens). one factor of the multiplication I rounded a little bit down, the other one a little bit up. therefore the total result should still be near that.
for 7 I did a similar thing. I rounded 975 to 980 and 23 to 20. just now we need to divide.