If size 6 pants require 25 inches of elastic and size 10 pants require 30 inches of elastic how much would size 12 pants need

32.5 in

Step-by-step explanation:

Going up four sizes (from size 6 to size 10) requires an extra 5 in of elastic.

The rate of increase is 5 in/4 sizes or 1.25 in/size.

To increase another two sizes (from size 10 to size 12), we must add another 2 * 1.25 = 2.5 in of elastic.

The length of elastic needed for size12 is 30 + 2.5 = 32.5 in.