The law is:

The first addend is increased by 5, 6, 7, 8, 9, ... and then multiplied by 1, while the 20th addend is 4+20=24.

The second addend is 7, 10, 13, 16, 19, ... and the 20th addend is 3x20+4=64.

So the 20th addition formula is 24+64.

The sum of their first 20 formulas is:

5+7+ 6+ 10+7+ 13+ 8+ 16+……+24+64

=(5+6+7+8+……+24)+(7+ 10+ 13+ 16+……+64)

=29x 10+7 1x 10

=290+7 10

= 1000