Sembazuru:
(Side note about the "beings with 10 fingers" comment... Back when I used to wait tables I developed a finger counting method that, while is actually base 5 (1,2,3,4,10,11,12,13,14,20, etc...), I called "groups of 5" that I used for counting number of chairs or people in a room. Quickly move my fingers behind my back, and then when back in the safety of the waiter's station or kitchen count 25 for the left thumb, 5 for each left finger, and 1 for each right digit. To the customers it just looks like I'm standing there looking over the room with my hands behind my back. My short-term memory for numbers is so short, I needed to keep track on my fingers so I didn't loose count if someone asked me where the bathroom is... So much for base 10 being default...)
There actually is a combination base-5 / base-10 method called "chisanbop".
Right fingers (except for thumb) = 1 unit per finger
Right thumb = 5 units
Left fingers (except for thumb) = 10 units per finger
Left thumb = 50 units
And as for losing track of numbers, I too am very distractable. Only for me it's not limited to numbers.
I wonder: why is it considered less socially acceptable to refuse to acknowledge someone who is interrupting you, than it is to interrupt someone? Why isn't the interrupter the "bad guy"?