The rectangle is 12 cm long and two-thirds wide. Find the perimeter of the rectangle.

In order to find the part of an integer value, you must divide it by the denominator of the fraction and multiply by its numerator.
Based on this rule, we find out what the width of the rectangle is, provided that it is 2/3 of its length, equal to 12 cm:

12: 3 * 2 = 8 cm.

We define the perimeter of such a figure, which corresponds to the sum of all its sides, according to the formula:
P = 2a + 2b = 2 * 12 + 2 * 8 = 24 + 16 = 40 cm.