The typical travel time during the morning commuting hours decreased

This is a question on Percentage decrease. The expression to find out Percentage decrease is,

Percentage Increase = \(\frac{Initial Value – Final Value }{ {Initial Value}}\) * 100.

Note that the question mentions the word approximate and therefore gives you a chance to hasten up proceedings by approximating the final answer. You do not have to break your back doing exact calculations.

In Data interpretation questions, it’s also important to develop the skill of estimating values shown on the graph. For example, in this graph, the time taken for the morning commute in 1986 is somewhere between 15 and 20 minutes. A close observation tells me that it’s almost half-way between 15 and 20. This means that we can take it as 17.5 since each interval is of 5 minutes. However, since the question asks us to find an approximate answer, there’s nothing wrong in assuming this to be 18 (17 is equally good for that matter).

In 1988, the time taken for the morning commute is approximately 13 minutes.

Substituting these values in the expression, we have,
Percentage Decrease = \(\frac{18 – 13 }{ 18}\) * 100 = \(\frac{5}{18}\) * 100.

This is exactly where you have to realise that going for the exact answer is not needed. If we had \(\frac{5}{15}\), the percentage would be 33.33% since \(\frac{5}{15}\) is nothing but \(\frac{1}{3}\). But \(\frac{5}{18}\) is smaller than \(\frac{5}{15}\), therefore, the percentage value should be less than 33.33%. This helps us eliminate options D and E since they are more than 33.33%.

The possible answer options at this stage are A, B or C. 5% of 18 is 0.9 and 10% is 1.8. But the number in the numerator of our expression is 5, which is way higher than either of these two. Answer options A and B can be eliminated.
The correct answer option is C.

Note that the data itself has been designed in such a way that approximation is certainly the better approach to solve this question. Even with a calculator in your arsenal, it would not be prudent to solve for the exact answer since we do not know the exact values for the 1986 and 1988 commute times.

Hope that helps!