1-digit natural numbers:

There are of course 5 1-digit numbers:

3 4 5 6 7

2-digit natural numbers:

We can choose the first digit any of 5 ways.
We can choose the second digit any of the 4 remaining ways,

That's 5·4 or 20 ways

They are

34 35 36 37 43 
45 46 47 53 54 
56 57 63 64 65 
67 73 74 75 76

Three digit numbers:

We can choose the first digit any of 3 ways, (3,4, or 5}
We can choose the second digit any of the 4 remaining ways,
We can choose the third digit any of the 3 remaining ways.

That's 3·4·3 = 36 ways.

Here they are:

345  346  347  354  356  357
364  365  367  374  375  376
435  436  437  453  456  457
463  465  467  473  475  476
534  536  537  543  546  547
563  564  567  573  574  576

Total: 5+20+36 = 61 ways

Edwin