777 maths problem

The digits 1-9 add up to 45 and 3x7=21. For every number "carried" you increase one place by one and decrease another by ten so a net -9 meaning you can get combinations that add up to 36 or 27 or 18 or 9 but not 21 so this is not possible.

Example 145, 267, 398 = 810 adds up to 9.

Engineer, I think, in your haste to asnwer, you answered a different question from the one that was asked.

There are many sets of 3 three-digit numbers that add up to 777. Is the question "how to calculate how many"?

Nope - he was spot on: "using all digits 1-9"

The digital root of the sum of any set of numbers made up of the digits 1-9 (used once each) will be 9 (or 0). The digital root of 777 is 3.