Is there a formula for calculating multiples

[taffmorgan7274]

Is there a formula for calculating the sum if the price of an item is £20 but on offer for 3 for £50. I would have the £20 price in cell A1 and I would like to find out a formula which would calculate if somebody purchased multiples of 3 or calculate a multiple of 3 and anything which is non multiple eg if a customer bought 5 items.

 

[bebo021999]

With each Qty 3, deduct 10
Qty = 3 , deduct 10
Qty = 4-5 , deduct 10
Qty = 6-8 , deduct 20
...

=Price*Qty-INT(Qty/3)*10

Assume A1 is Price; A2 is Qty

=A1*A2-INT(A2/3)*10

 

[taffmorgan7274]

With each Qty 3, deduct 10
Qty = 3 , deduct 10
Qty = 4-5 , deduct 10
Qty = 6-8 , deduct 20
...

=Price*Qty-INT(Qty/3)*10

Assume A1 is Price; A2 is Qty

=A1*A2-INT(A2/3)*10

Thank you very much for this, it will save the aching brain cell!