17, 18, 19, 21, 22, and 23
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You have seven different digits (symbols) to choose from, so you can form seven different one digit numbers and 7×7=72=49 different two digit numbers.
There are 8*8 = 64 such numbers.
what is the least possible sum of two 4-digit numbers?what is the least possible sum of two 4-digit numbers?
Well, friend, when you add two two-digit numbers together, the sum will always be a two-digit number or a three-digit number, not a four-digit number. It's like mixing different colors on your palette - you'll get a beautiful new shade, but it won't be something completely different. Just keep adding those numbers together and creating something wonderful!
81 As there are no limits stated then you can have a number comprising a repeated single digit (such as 2222), two pairs of numbers (e.g. 2244) or three different numbers (such as 2462). The first digit can be one of any of the 3 numbers. The second digit can be one of any of the three numbers, as can the third digit and also the fourth. Then you can have 3 x 3 x 3 x 3 = 81 different 4-digit numbers using the three given numbers.