You can force Solver to use integers by adding an int constraint.
Using Solver and adding constraints is covered in Expert Skills Lesson 7-6: Use Solver.
Simply apply a constraint to the 'changing' cells (ie. the cells that Solver is going to change) with settings similar to the following:
This will force Solver to use only whole numbers in the specified cells.
I can provide a more specific answer if you'd like to attach a copy of your spreadsheet or a screenshot of your Solver settings.
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