How to calculate 2800-2-4-6-8. - 98-100 with a simple method in the fourth grade?

How to calculate 2800-2-4-6-8. - 98-100 with a simple method in the fourth grade?

2+98=100
4+96=100
6+94=100

And so on, from 1 to 100, there are 25 100
2800-25*100-100=200
(because I only calculate 98, and I still have a 100, so I have to subtract 100.)