unable to change variable values in Xcode playground

I am just starting out in Xcode6 and using Playground to test out some Swift commands. I test a simple var a = 10, var b = 15 and var total = a+b So far so good. But when I change value of a from 10 to 20, it doesn't seem to be updated. The calc. still reflects 10+15 vs. 20+15. This sounds silly but how come variable values aren't updated? Should I be using different variables or doing something else? I would have expected the 'right panel' to update instantly with my changes. Thanks!