Second attachment 👇
It looks like you want to divide *each* of the values in the {Quantity} lookup field by the rolled up {Count}. However, in Airtable, math doesn't work when there are multiple values in a lookup field.
You can use a system of back-and-forth rollups to pass the {Count} down to the linked table. Then each linked record can divide its {Quantity} by the total {Count}. Finally, lookup the quotient from the linked records. This system does not work for some base schemas but works quite well in others, especially when there is only handful of linked records for each main record.
On the other hand, if you want to divide the *sum* of the {Quantities} by the {Count}, you could turn the {Quantity} into a rollup field with the formula SUM(value). Then you would have only one value in the {Quantity} field.