The bill was for $318, including a 6% tax. Notice that 6% is added to the net cost - that is, the tax is not 6% of 318, but 318 is 106% of the electricity cost. This means 318 is (106/100) = 1.06 times the net electricity cost.
In order to isolate electricity, we will divide each side by 1.06.
That means that:
Electricity × 1.06 = 318
318 ÷ 1.06 =
(318 ÷ 106) × 100 =
3 × 100 =
300
💡 Solving tip
Before trying to solve the question, it may be helpful to first look at the answer choices. Notice that according to answer A the price without tax ($337.08) is higher than the price with tax ($318). Since this cannot be, answer A can be eliminated.
According to answer B, the price without tax ($19.02) is almost $300 less than the price with tax ($318). We know that 6% of 19.02 cannot be anywhere near 298.92 and we can therefore eliminate answer B.
The two remaining answer choices are very close together, $298.92 and $300. Instead of dividing 318/1.06, we can multiply one of these two answer choices by 1.06 and see if we get 318.
We will check 300, as it is a much more comfortable number to work with.
300 × 1.06
= (3 × 100) × (106 ÷ 100)
=3 × 106
=3 × 100 + 3 × 6
= 300 + 18 =
318
300 is therefore the correct answer. Notice that if we would have first multiplied 298.92×1.06 we would have gotten 316.8552. We could have then eliminated answer D and concluded answer C was the correct answer without checking it.