Explanation:
Draw a rectangle. Then draw three vertical lines inside to split the rectangle into 4 equal pieces. Shade one of the pieces to represent 1/4.
Now draw 2 horizontal lines to split that 1/4 piece into three equal smaller pieces. You should end up with a 3 by 4 grid of small squares. There are 3*4 = 12 small squares total.
Each person gets 1/12 of a unit of ribbon, when we started off with 1/4 of a unit of ribbon. Notice how 3*(1/12) = 3/12 = 1/4.
Refer to the diagram below.