Problem:

Column A contains a list of strings that are composed of two numbers separated by a \"/\" symbol.
We want to add 10 to the left number of each string.

Solution:

Use the LEFT, FIND, and MID functions as shown in the following formula:
=LEFT(A2,FIND(\"/\",A2)-1)+10&MID(A2,FIND(\"/\",A2),255)