28 + 30 + 32 + .... + 86
Note....that we can pair the terms as follows
[ 28 + 86 ] + [ 30 + 84] + [ 32 + 82 ] +......+ [56 + 58]
Each pair sums to 114
And the number of pairs is given by [56 - 28] / 2 + 1 = 15 pairs
So...the sum is 15 * [ 114 ] = 1710