Official Explanation
Analyze the information
This is a Graphics Interpretation question. All answers are based on information from the graph provided. The graph is a stacked bar chart with a bar for each of seven days. From the bottom up, each bar indicates the number of breakfast customers, the number of lunch customers, and the number of dinner customers.
The opening paragraph also makes it clear that customers are not assigned to more than one group. Grouping is based solely on when a customer is seated. So, if a customer is seated during breakfast hours (say, 11:30 am) and leaves during lunch hours (say, 12:30 am), that person is included in the breakfast count, regardless of what that person orders.
Approach strategically
1. The day on which customers seated during breakfast hours made up the smallest percentage of that day's total customers is___________
This question asks for the day when breakfast customers made up the smallest percentage of the day's total. Instead of calculating the percentage for each day, use the drop-down menu to narrow down which days to test. Only Sunday, Tuesday, Wednesday, and Saturday need to be considered.
Some visual comparison of the bars in the chart can save a lot of calculation. If any days have about an equal number of total customers, more breakfast customers would constitute a higher percentage. (For instance, 20 out of 100 is a greater percentage than 10 out of 100.) The total customer counts for Sunday and Saturday are about equal, but Sunday's breakfast count is almost double Saturday's. Thus, Sunday's breakfast percentage is higher. That eliminates Sunday as an option.
Similarly, for days with similar breakfast counts, the percentage of total customers is smaller when the total number of customers is noticeably larger. (For instance, 10 out of 100 is a smaller percentage than is 10 out of just 50.) The remaining days—Tuesday, Wednesday, and Saturday—have very similar breakfast counts (89, 85, and 91, respectively). However, Saturday's total customer count (over 500) is substantially larger than either Tuesday's or Wednesday's (under 400 each). So, Saturday is the day with breakfast constituting the smallest percentage of total customers.
If you weren't comfortable using estimation, this answer could be found using the actual figures. For Sunday, breakfast is 176 out of the total 176 + 185 + 140 = 501 customers, for a percentage of \([m]176/501\)≈35[/m]%. For Tuesday, breakfast is 89 out of the total 89 + 150 + 129 = 368, for a percentage of \(\frac{89}{368}≈24\)%. For Wednesday, breakfast is 85 out of the total 85 + 162 + 118 = 365, for a percentage of \(\frac{85}{365}≈23\)%. And for Saturday, breakfast is 91 out of the total 91 + 191 + 231 = 513, for a percentage of \(\frac{91}{513}≈18\)%.
TAKEAWAY: Remember that a percentage is a ratio comparing the relative values of two quantities. When values are presented visually, as in a graph, you can often compare percentages by comparing the relative height of bars (or size of slices in a pie chart, etc.) without calculating the actual values.
2. On exactly__________day(s), the number of customers during breakfast hours was greater than the average number of customers per day during dinner hours for the week.
The question asks for the number of days for which the breakfast count is higher than the average daily dinner count. Each day's dinner count is at least 118, so the average daily dinner count must be higher than 118. Only Sunday's breakfast count of 176 is above 118, so it is the only one that could possibly exceed the average dinner count. Eliminate (C) and (D).
Without calculating the actual average dinner count, a little estimation and understanding of the balance approach to averages could save a lot of time. If the dinner average were 176, the total number of customers below average on below-average days would equal to the total number of customers above average on above-average days. However, the first five days have dinner customer levels ranging from 118 to 140. That's more than 30 customers below average each day, for a total of more than 150 customers below average. The remaining days, Friday and Saturday, are, respectively, less than 30 and 60 dinner customers above the hypothetical average of 176, for a total of less than 90 customers above that average. This is not nearly enough to balance out the below-average days. So, the dinner average must be lower than 176. That means Sunday's breakfast count of 176 is higher than the daily dinner average, making Sunday the one day with a large-enough breakfast count.
For the record, the average daily dinner customer could be calculated. Throughout the week there are 140 + 137 + 129 + 118 + 133 + 204 + 231 = 1,092 dinner customers. Over 7 days, that's an average of \(\frac{1,092}{7}=156\) dinner customers per day. And, sure enough, Sunday is the one and only day with a breakfast count above 156.
TAKEAWAY: When a question asks about an average, see whether you can use the balance approach to determine whether the average is above or below a certain amount, instead of calculating the actual value.