The length of the rectangle is 4 cm. What should be the width so that the perimeter of the rectangle is less than 20 cm?

Let us determine, by writing down the inequality, what the width of a rectangle can be so that its perimeter is less than 20 cm, with a length = 4 cm.

The formula for finding the perimeter of such a figure corresponds to:

P = 2a + 2b.

Substitute the known values:

2 * 4 + 2 * b <20.

On the left side, we will perform the actions, leaving the variable with the coefficient, and transfer the known value to the right side, changing the sign in front of it to the opposite:

2b <20 – 8.

2b <12. The second factor is the quotient of the first and therefore:

b <6.

Width should be less than 6 cm.