Forms :: Calculating Field Value

calculating a field value.I've got 5 products, numbered 1 - 5 in tblProducts.productID..In one order I'll only ever have the 1 - 5 products, with a maximum of 5 products in any Order.

products
1 = mushroom
2 = tomato
3 = egg
4 = fish
5 = meat

tblOrder.orderID
tblOrderDetail.productID
I have a field in tblOrderDetail called "productNumber" which is where the result should go of the calculation.

I need to calculate the total number of products ordered but it could be a combination of them, the thing is that I need to reference them in a specific order if they exist in the order as per the schedule above.

example
order 1 = products 1,3,4

the result should be
1.3 (mushroom)
2.3 (egg)
3.3 (fish)

example 2
order 2 = products 1,4

the result should be
1.2 (mushroom)
2.2 (fish)

example 3
order 3 = products 4,1 (this one shows that we haven't ordered it in order, I still need to display that correct order)

the result should be
1.2 (mushroom)
2.2 (fish)