The average of 330 and 410 is 370.
330
No number between 55 and 101 is a multiple of 410, though some are multiples of 20. 60, 80 and 100 are the multiples of 20 in that range, but none of them are multiples of 410. So no number between 55 and 101 is a multiple of 410 and 20.
380 and 410
This is expressed as 250< n < 330 and covers any number in the range 251 to 329. If 250 and 330 are to be included in the range then the expression is written 250 ≤ n ≤ 330.
You can calculate the number halfway between 250 and 330 by adding them together, then divide by 2. For example, 250 + 330 = 580, and 580 divided by 2 = 290, which is the number halfway between the two numbers.
An infinite number ofd them. 251, for example, or 251.3256.
330 is one such number.
The number 330 is "three hundred thirty."
410 is, indeed, a decimal number.
410 = 401 + 9
330