Open Array Decimal Multiplication Tool. The Open Array Decimal Multiplication tool is used to demonstrate how decimal multiplication can be represented in various ways. Students choose from six renderings of the open array model that accommodate 2 digit x 2 or 3 digit problems and 1 digit x 1, 2, 3, or 4 digit problems. The standard algorithm is presented with a flexible ability to articulate and add partial products. Word problems can be entered to anchor each problem.