Change one of the $+$ signs to an equals sign so that you create an addition problem with a true answer:
$$\text{1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20}$$
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Sign up to join this communityAssuming the 2- is actually 20...
1+2+3+4+5+6+7+8+9+10+11+12+13+14
=
15+16+17+18+19+20
Easy:
put '=' in between 14 and 15 ie
1+2+3+4+5+6+7+8+9+10+11+12+13+14=15+16+17+18+19+20
Easy way to find out N(N+1)/2 =
(20*21)/2 - N(N+1) /2