To solve this kind of challenge, sometimes it just needs logical steps and other times it involves lateral out-of-the-box thinking!

## Challenge:

In how many 3 digits number the sum of all digits is 7?

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The correct answer is 28.

First, let’s find all three digits that add up to 7.

700, 610, 511, 520, 430, 421, 322, 133

For 7, 0 and 0, we can write only one three digit number which is 700.

For 6, 1 and 0, we can find 106, 160, 601, 610,

For 5, 1, and 1: 115, 151, 511

For 5, 2, and 0: 205, 250, 502, 520

For 4, 3, and 0: 304, 340, 403, 430

For 4, 2, and 1: 124, 142, 214, 241, 412, 421

For 3, 2, and 2: 223, 232, 322

For 1, 3, and 3: 133, 313, 331

There are 28 three digits numbers that the sum of their digits is 7.