Calculate the area of a rectangle with side 4cm and 5cm. Your answer must be in cm. 2. Calculate the area of a square if its side is 3m. 3. Find the area of a square if the diameter is 30cm.

The area of a rectangle is given by the formula:

$$A = \text{length} \times \text{width}$$

Given length $= 5 \text{ cm}$ and width $= 4 \text{ cm}$.

$$A = 5 \text{ cm} \times 4 \text{ cm}$$

$$A = 20 \text{ cm}^2$$

The area of the rectangle is $\boxed{\text{20 cm}^2}$.

2. Calculate the area of a square if its side is 3m.

The area of a square is given by the formula:

$$A = \text{side} \times \text{side} = \text{side}^2$$

Given side $= 3 \text{ m}$.

$$A = (3 \text{ m})^2$$

$$A = 9 \text{ m}^2$$

The area of the square is $\boxed{\text{9 m}^2}$.

3. Find the area of a square if the diameter is 30cm. $\pi = 3.14$

Assumption: The question likely intended to ask for the area of a circle given the diameter and the value of $\pi$. A square does not have a diameter in the same way a circle does. If it were a square, $\pi$ would not be used.

The radius $r$ is half of the diameter.

$$r = \frac{\text{diameter}}{2} = \frac{30 \text{ cm}}{2} = 15 \text{ cm}$$

The area of a circle is given by the formula:

$$A = \pi r^2$$

Given $r = 15 \text{ cm}$ and $\pi = 3.14$.

$$A = 3.14 \times (15 \text{ cm})^2$$ $$A = 3.14 \times 225 \text{ cm}^2$$

$$A = 706.5 \text{ cm}^2$$

The area of the circle is $\boxed{\text{706.5 cm}^2}$.

4. Calculate the area of a triangle if the base is 28mm and the height is 15mm. Your answer should be in mm.

The area of a triangle is given by the formula:

$$A = \frac{1}{2} \times \text{base} \times \text{height}$$

Given base $= 28 \text{ mm}$ and height $= 15 \text{ mm}$.

$$A = \frac{1}{2} \times 28 \text{ mm} \times 15 \text{ mm}$$

$$A = 14 \text{ mm} \times 15 \text{ mm}$$ $$A = 210 \text{ mm}^2$$

The area of the triangle is $\boxed{\text{210 mm}^2}$.