1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 15
= (1+9)+(2+8)+(3+7)+(4+6)+ ( 5+15 )
= 10+10+10+10+20
=60
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 15
= ( 1 + 9 ) + ( 2 + 8) + ( 3 + 7 ) + ( 4 + 6 ) + ( 5 + 15 )
= 10 + 10 + 10 + 10 + 20
= 40 + 20
= 60
= ( 1 + 9 ) + ( 2 + 8 ) + ( 3 + 7 ) + ( 4 + 6 ) + ( 5 +15 )
= 10 + 10 + 10 + 10 + 20
= 40 + 20
= 60