I sell the same product in multiples of 1, 10 and 20. how do I deduct 10 or 20 from the stock?

[Benmc]

hi

I sell the same product in multiples of 1, 10 and 20. how do I deduct 10 or 20 from the stock?

the product is set up with 3 varients, a single tub, a box of 10 tubs and a box of 20 tubs.

how do I account for all 3 variants with 1 stock take? 

if I have 100 tubs and sell a box of 20 I need to stock to go down to 80, if I then sell 1 tub I need the same stock take to go down to 79....?

all 3 variants need to be connected to the same stock count.

any help is appreciated ?

thank you.