Anička and Blanka each wrote a two-digit number starting with a seven. The girls chose different numbers. Then each inserted a zero between the two digits, creating a three-digit number. From this, each subtracted their original two-digit number. The result surprised them.

Determine how their results differed.

(L. Hozová)

Hint. Try the described procedure with several specific numbers.
A two-digit number starting with a seven is of the form $7 *$, where the asterisk can be any digit. By inserting a zero, we get a three-digit number of the form 70*. Regardless of which digit the asterisk represents in the units place, the difference is

$$
\begin{array}{r}
70 * \\
-\quad 7 * \\
\hline 630
\end{array}
$$

The results of Anička and Blanka did not differ; both got 630.