Permutation And Combination

Step 1: Fixing digits 3, 4, and 5 in the middle.
Since the digits 3, 4, and 5 are always to be in the middle, they are fixed in three positions in the middle of the number.
Step 2: Choosing other digits.
The remaining positions will be filled by the remaining digits: 1, 2, 6, and 7. These digits can be arranged in (4 factorial) ways. Step 3: Conclusion.
Thus, the total number of natural numbers that can be formed is since there are 6 ways to arrange the digits 3, 4, and 5 in the middle positions.