Visual Expressions Worksheets - Math Worksheets Center Practice problems are provided. Write the numerical expression for the models. Find the sum. Let one black tile equal +1 and one white tile equal -1. The black tile is equal to +1. There are 10 black tiles the value is 10. The white tile is equal to -1. There are 6 white tiles the value is -6. The sum will be 10+ (-6) = 4.