Determine the sum of all distinct $4$-digit numbers formed by $2, 3, 7, 9$

Question

Determine the sum of all distinct $4$-digit numbers formed by $2, 3, 7, 9$

Attempt: Each number appears in each digit place $6$ times, so the answer should be $$6\times\underbrace{(1000+100+10+1)}_{\text{to account for each digit place}}\times\underbrace{(2+3+7+9)}_{\text{to account for each number}}=\boxed{139986}$$

Answer 1

Your approach and answer are correct.