Radio Astronomy Systems Research Group

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) (Richard's method) 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.