Let h = the hundreds digit (of Mico's house number)
Let t = the tens digit 
Let u = the units digit

So the house numbers written in digit form are 

Mico's house number = htu.
Ben's house number = tu.
Clara's house number = u.

We form the arithmetic (basic math) addition:

htu
 tu
  u
---
912

In the right column To get the 2 at the bottom, the 3 u's have 
to be 4 so they'll add up to 12 and there'll be 1 to carry.  So 
we have this:

 1
ht4
 t4
  4
---
912

Looking at the middle column of digits, either the two t's are 0 
so the middle column of digits will add up to 1, or the two t's 
are 5 so that the middle column will add up to 11 with 1 to carry.
But t can't be 0 because that would make Ben's house number 04.  
But nobody writes 04, they just write 4.  So if you removed the 
leftmost digit of 4 you would have nothing at all!  So 0 is out.

Therefore t = 5  (That's the answer).

Completing the addition:

11
h54
 54
  4
---
912 

Since we have 1 to carry to the left column,
which gives us a 9 at the bottom left, h can 
only be 8.

11
854
 54
  4
---
912

The house numbers are 854, 54, and 4 and the 
middle digit of Mico's house number is 5.

Edwin