A 30cm board is cut down to be 25.5 cm. What is the percent decrease in length?

Okie, so percent decrease is the amount loss divided by the original number and multiplied by 100 to find the percent. This is because you are finding the amount loss in terms of the total amount - percent basically.
The amount loss : 4 feet
4 feet/ 12 feet = .3333 (ratio of how much cut) multiply by 100 --> 33.3% decrease. From 12 feet to 8 feet is a 33.3 % decrease. I hope this helps :-)