Bitshifting to multiply an integer by 10

Question

Simple question, but I can't quite seem to figure it out:

If i have an integer, say 12, and I perform the following bit-manipulation on it:

int i = 12;  
i = (i << 3) + (i << 1);

I end up with 120 (12*10). This is the case with any number.

Can someone explain to me, succinctly, why it is that this works? (I'm obviously missing something quite rudimentary when it comes to bitshifting).

Answer 1

Express as multiplication.

i = (i << 3) + (i << 1);
i = (i * 8) + (i * 2);
i = 8i + 2i
i = 10i