Question

这数学怎么做？

题目：How many whole numbers from 0 to 1000 have digits with a sum of 10?
-
我要的是步骤，不要直接写答案。。
还有，解释为什么要怎么做，老师要我们解释的。

Answer 1

for one digit number, there is 0;

for two digits numbers, there are 9 which sum of digits is 10;
because for every tens from 1 to 9, there is only one two digits number meets that condition.

for three digits numbers, if hundreds are:
1, there are 10 numbers
2, there are 9 numbers
3, there are 8 numbers
4, there are 7 numbers
5, there are 6 numbers
6, there are 5 numbers
7, there are 4 numbers
8 , there are 3 numbers
9, there are 2 numbers

Why are there 10 numbers when hundreds is 1?
because 0-9 can be tens, and when tens fixed, this three digit number is fixed.

when hundreds is 2, only 0 - 8 can be tens;
same as 3 - 9.

Total is: 9 + 10+9+8+7+6+5+4+3+2
63

Answer 2

晕倒，整数翻译成whole number，不知道是哪个家伙拍脑袋的想法。如果是老师，那这个老师肯定有问题。