7.2. Find the sum of all three-digit natural numbers that do not contain the digit 0 or the digit 5.
A number or a short expression. Spacing, $ signs and \frac vs / are all fine.
Official solution
Answer: 284160. Solution. We will add the numbers in a column. Each last digit appears in the units place as many times as there are three-digit numbers ending with this digit. Therefore, it will appear 8⋅8=64 times (since a total of 8 digits are used for the hundreds and tens places). Thus, the sum of the digits in the last place is 64⋅(1+2+3+4+6+7+8+9)=2560. Similarly, in the tens and hundreds places, we get the same sum. In the end, we get 2560⋅100+2560⋅10+2560=2560⋅111=284160.
Source: NuminaMath-1.5,
licensed Apache-2.0.
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