If the total number of parts is 12 and you want to reduce the dish to two-thirds its current size, the number of parts that will be reduced is (1 - 2/3) = 1/3
To reduce 1/3 of the 12 parts, you need to multiply 12 by 1/3 to know how many parts is that:
12 * 1/3 = 12/3 = 4
You need to subtract 4 parts. If you have 4 ingredients, you can remove 1 part of each, so each ingredient now will have 2 parts.
