Use two complete lines

A number matrix is a small grid with one missing number. The instruction tells you whether to compare rows or columns. Read each row left to right, or each column from top to bottom.

Look at a complete line first. Try a simple arithmetic relationship between its numbers. Then check that same relationship on the other complete line. A rule that fits only one example is not enough.

In this example, compare rows. The first row gives 2, 3, 5; the second gives 4, 2, 6. What calculation works in both?

Adding the first two numbers gives the third: 2 + 3 = 5. Check the next row in the same direction: 4 + 2 = 6. The relationship works twice.

Apply it to the incomplete row: 3 + 4 = 7. The missing number is 7.

Other questions use different relationships. Do not carry this addition rule into a new grid without checking its complete lines. Keep the operation and any constant consistent within that question.

Compare rows again. The complete rows are 3, 4, 7 and 5, 3, 8. Check the relationship against both before using the last row, 6, 5, missing.

This lesson gives two choices to check the method. The practice gives five choices for each missing number.