Question

20+21+22+...+29+30.

Answer 1

so so hang la :

(30-20):1+1=11(so)

tong do la :

(30+20)*11:2=275

Answer 2

20+21+22+23+24+25+26+27+28+29+30

=(20+30)+(21+29)+(22+28)+(23+27)+(24+26)+25

=50+50+50+50+50+25

=50x5+25

=250+25

=275