84, 168, 252, 336, 420 +84 . . .
21 and 28 are composite.
When figuring out what number goes both into 21 and 28, you should look at the factor paris. 21: 1 x 21, 3 x 7 28: 1, 28, 2 x 14, 4 x 7 7 goes into both 21 and 28.
28
84
The median is 27.
To find the number halfway between any two numbers x and y, add x and y, then divide the answer by 2. So for 21 and 35, 21+ 35 = 56. 56/2 = 28. You can check your answer by seeing that 21 + 7 = 28 and 35 - 7 = 28.
Number of neutrons = mass number - atomic number = 49 - 21 = 28
28
In this sequence, it should be 27, not 28 (28 is not divisible by 3)
28
1 and 7.
His favorite number is 21