Question

is this a combination problem ?


1 1 2 1 2 3 1 2 3 4 1 2 3 4 5 . . .
The number above begins with a "1", then continues "12," "123," "1234," and so on until the number "9" is reached. How many digits are in the final number?
A. 36
B. 40
C. 45
D. 54
E. 81
Correct Answer: C


i know the solution is to add the numbers together and then you get 45 , but i don’t understand why ?

why should we add the numbers together instead of multiplying ?

what type of problem is this ?

is this a combination problem ?