A relationship that states that two ratios or two rates are equal.

If \(10\) beans have a mass of \(17\text{ g}\), then \(30\) beans have a mass of \(51\text{ g}\).

Thus, the proportion is,

\(\dfrac{17\text{ g}}{10\text{ beans}}=\dfrac{51\text{ g}}{30\text{ beans}}.\)

The electricity costs \(43.6\text{ sen}\) for \(2\text { kilowatt-hour (kWh)}\).

How much does \(30\text{ kWh}\) cost?

i) Unitary method

The cost of electricity for \(2\text{ kWh}\) is \(43.6\text{ sen}\).

So, the cost of electricity for \(1\text{ kWh}\) is

\(\begin{aligned}&=\dfrac{43.6\text{ sen}}{2} \\\\&=21.8\text{ sen}. \end{aligned}\)

Thus, the cost of electricity for \(30\text{ kWh}\) is

\(\begin{aligned}&=30\times21.8 \\\\&=654\text{ sen}. \end{aligned}\)

ii) Proportion method

Let the cost of electricity for \(30\text{ kWh}\) be \(x\text{ sen}\).

\(\begin{aligned} \dfrac{43.6\text{ sen}}{2\text{ kWh}}&=\dfrac{x\text{ sen}}{30 \text{ kWh}} \end{aligned}\)

We can see that 

\(2\text{ kWh}\times15=30\text{ kWh}.\)

Thus,

\(\begin{aligned}x&=43.6\times15 \\\\&=654. \end{aligned}\)

iii) Cross multiplication method

Let the cost of electricity for \(30\text{ kWh}\) be \(x\text{ sen}\).

\(\begin{aligned}\dfrac{43.6}{2}&=\dfrac{x}{30} \\\\2\times x&=43.6\times30 \\\\x&=\dfrac{1\,308}{2} \\\\&=654. \end{aligned}\)

From these methods, the cost of electricity consumption for \(30\text{ kWh}\) is \(\text{RM}6.54\).