Question 237583
There are 10 2 digit numbers that start with 1 (10,11,12...19)
There are 10 2 digit numbers that start with 2 (20,21,...29)
Do this all the way up to the 10 2 digit numbers that start with 9 (90,91...99)

Now sum them
10*1= 10
10*2=20
10*3=30
...
10*9= 90
So you can see that sum is 10 + 20 + 30 + 40 + ...+ 80 + 90
Adding them up, you get 450