Because all of those numbers are divisible by 21.
You could add 21 each time. Or write the 21 times table. Or use a spreadsheet. All three methods will prove that 21, 42, 63, 84, and 105 are multiples of 21.
21,63,84,105,?
The first five multiples of 21 are: 21, 42, 63, 84, 105.
21, 42, 63, 84, 105.
42 63 84 105 126 147
21, 42, 63, 84, 105, 126, 147, 168, 189, 210
21, 42, 63, 84, 105, 126, 147, 168, 189, 210
The first 5 multiples of 21 are: 21, 42, 63, 84, and 105.
The first five multiples of 21 are: 21, 42, 63, 84, 105.
The first 5 multiples of 21 are: 21, 42, 63, 84, and 105.
21, 42, 63, 84, 105.
21, 42, 63, 84, 105, 126 . . .
21,42,64,86,108,130,152,174,196,218They are 21, 42, 63, 84, 105, 126, 147, 168, 189, 210.
Factors of 21 = 1, 3, 7, 21 Multiples of 21: 21, 42, 63, 84, 105, 126, 147 +21 . . .
42 63 84 105 126 147
Factors: 1, 3, 7, 21. Multiples: 21, 42, 63, 84, 105, 126, 147, 168, 189, 210 +21 . . .
21, 42, 63, 84, 105, 126, 147, 168, 189, 210, 231, . . .
21, 42, 63, 84, 105, 126, 147, 168, 189, 210.
21, 42, 63, 84, 105, 126, 147, 168, 189, 210, 231 . . .