Let K of J be defined as the sum of all integers between K and J. For example 5 of 9=6+7+8=21. What is the value of (80 of 110)-(81 of 109)?

Since these are fairly easy to do, we can write it out...
(80 of 110)-(81 of 109)=
(81+82+83+...+109) - (82+83+84+...+108)=
81+109=
190