How do I use HP 10bII+ to calculate common logarithms (is the logarithm with base 10)?

just calculate the natural log of your number, and then divide it by the natural log of the new base. Log(X) = log10(X) = LN(X) / LN(10)

For example to get Log(3) which is 0.47712125472

Press 3 [Orange Shift] [LN], the screen display 1.0986

Press [÷] 10 [Orange Shift] [LN], the screen display 2.3026

Press [=], the screen display 0.4771

The common logarithm is the logarithm with base 10. It is also known as the decadic logarithm and decimal logarithm. It is indicated by log10(x), or sometimes Log(x) with a capital L.