In how many whole numbers between 100 and 999 is the middle digit equal to the sum of the other two digits?

9 answers

45

Here's why:
If first digit is k, middle is k+i, last is i.

Max k+i is 9, so i ranges from 0 to 9-k

So, there are
9 numbers starting with 1,
8 starting with 2,
...
1 starting with 9

sum of numbers from 1 to 9 is 9*10/2 = 45
45
I LOVE MATHS 5+5=10
45
How many whole numbers between 100 and 999 have their digits in strictly decreasing order
198
53. Easy.
ZXedv3c1 niol8
45 btw do u like Olivia Rodrigo?