Find the algebraic expression for the numbers got by adding $10$ repeatedly to $9$.

Look at these patterns together:

$11, 21, 31, 41.....$

$19, 29, 39, 49.....$

Add the numbers in the same position as the first two patterns. Why do we get only multiple of $10$? Do we get all multiples of $10$ like this?

Take any three consecutive natural numbers and add the first and last. Is there any relation between this sum and the middle number?