(91 + 92 + 93 + . . . . . . + 110) is equal to

$$\eqalign{ & 1 + 2 + 3 + ...... + n = \frac{{n\left( {n + 1} \right)}}{2} \cr & \therefore 91 + 92 + ...... + 110 \cr & = \left( {1 + 2 + ...... + 110} \right) - \left( {1 + 2 + ...... + 90} \right) \cr & = \frac{{110 \times 111}}{2} - \frac{{90 \times 91}}{2} \cr & = 6105 - 4095 \cr & = 2010 \cr} $$