Final answer:
The correct answer is c. 22, as adding this number to the set would position it as the fourth number in the ordered list, resulting in a median of 23.
Step-by-step explanation:
The student asked which number could be included in the set 18, 27, 25, 20, _____, 21, 26, so that the median becomes 23.
To solve this, we first need to order the given numbers: 18, 20, 21, 25, 26, 27. There are six numbers, so when we insert a new number to make a set of seven, the median will be the fourth number in the ordered list.
For the median to be 23, the fourth number in this list must be 23. As 21 and 25 are the current third and fourth numbers, respectively, we can determine that the missing number must be 22 or 23 to make 23 the median.
Since 23 is already part of the options given, and it doesn't make 23 the median if added, 22 is the number that fits and achieves the desired median.
Therefore, the answer is c. 22