Subtraction

Called mummy as I dun know how to do subtraction, number 20 was too big, I dun have 20 fingers.

Z : Mum, can you help me when you come back ? I can’t do it because I dun have 20 fingers
M : Hmmm… how many toes do you have on your right leg ?
Z : 5
M : How many toes do you have on you left leg ?
Z : 5
M : 5 + 5 = how many ?
Z : 10
M : How many fingers do you have on you right hand ?
Z : 5
M : How many fingers do you have on you left hand ?
Z : 5
M : 5 + 5 = ?
Z : 10
M : 10 + 10 ?
Z : 20 !
Z : Ok, thanks. Bye…
M : ????? (do I really know)