The official way to convert C to F is to multiply C by 1.8, and then add 32. But if you'd like a quick way to get a rough guesstimate and give your brain a break from multiplying by 1.8, you can instead take your C reading, double it, and then add 30. It'll get you within a few degrees. (20 degrees C * 1.8 = 36, add 32, = 68 degrees. Quick way: 20*2 = 40, plus 30 = 70 degrees.)
To go from F to C, you take F, subtract 32, then divide by 1.8. Or the easy way: Subtract 30, then take half.