Use your third grade math solving techniques ... remember those "word" math problems you thought you would never use ? For example ....
3 parts individually sell for $2, $3 & $5 for a total of $10 when you bought them
% for each is 20, 30 & 50
If you bought the lot for a discount say $8 then the purchase cost for each is $1.60, $2.40 & $4
