How do I + and - to a value

How i can add and subtract values?

For example:

Initial value 100

After X seconds (or other event) add 40 .............. 100+40= 140

After X seconds (or other event) subtract 30 ........140-30= 110

Thanks.

If it is an instance variable you would go to the object related to it in the events.

If it is a global variable then it is in the system part of the events.