What is the median of this data set?

21, 42, 63, 84, 105, 126, 147

Mean: Mean Is You add all and you divide then by how many numbers you have Like 21+42+63+84+105+126+147=504

504:7=72

Medium: Is the Middle Number of the Set

So whats the Middle number of the set?

21, 42, 63, 84, 105, 126, 147

84 Is in the Middle of the set

Mode: is when the Number is Repeated different times in the set

But in this set we don't have Multiple of the the same Number so heres an Example

13, 13, 13, 13, 14, 14, 16, 18, 21

So you see 13 Is the Most common Number for so 13 is the Mode.

Which for your answer (Median) The Answer is 84.