1 + 3 + 5 + … + 97 + 99 = 2500

There are a few different tricks to this one:

You use summation notation and see that it simplifies to N^2 (where N is 50) (as Richard did)
You can use the Gaussian addition trick of adding pairs (1 + 99 = 100, 25 pairs so 100*25 = 2500) (Jay)
Or, you can do the Dr. Warnick and ask Siri.