Multiplying by 5: mult by 10 divide by 2

A very similar concept for tipping about 20% is taking 1/10th of the bill by moving the decimal one to the left and doubling that. To make it even easier for me I just round to the nearest $10 amount first.

Bill: $66.20 -> move left and round up = $7.00 and double to $14.00. The exact 20% amount is a little over $13 but I tend to round up because it is also faster to add whole dollars to the bill.