Ok, so his goal is to read 1,000 pages in 9 weeks. We know 1 week has seven days.
So let’s start with the first one. He reads 10 pages per day on the first week. That’s 10 x 7, which he read 70 pages for the first week.
As for the second week, it’s the same: 15 x 7 = 105 pages read.
Third week: 20 x 7 = 140 pages read.
Add: 70 + 105 + 140 = 175 + 140 = 315 pages read.
Subtract: 1,000 - 315 = 685.
That’s 685 pages he still needs to read until he hits his goal. Hope this helped! :D