When you encounter questions asking for the mean (average) of a data set, you need to add all values and divide by the number of data points. This is one of the most fundamental statistical calculations you'll see on quantitative reasoning exams.

To find the mean number of patients per week, first add all six weekly totals: $$18 + 21 + 24 + 19 + 27 + 21 = 130$$. Then divide by the number of weeks: $$\frac{130}{6} = 21.666...$$ which rounds to 21.7.

Looking at the wrong answers: Choice B (22.0) might result from rounding errors or miscalculating the sum as 132 instead of 130. Choice C (21.0) could come from mistakenly using the mode (the most frequently occurring value, which is 21) instead of the mean, or from premature rounding. Choice D (22.5) likely results from a calculation error, perhaps getting a sum of 135 and dividing by 6.

The key trap here is confusing mean with other measures of central tendency. The mode is 21 (appears twice), but the question specifically asks for the mean. Also, be careful with your arithmetic—double-check your addition and don't round until the final step.

Remember: mean questions on the DAT are straightforward calculations, but accuracy in arithmetic is crucial. Always verify your sum before dividing, and round only your final answer to match the precision of the given choices.