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The table categorizes 50 African countries according to GDP (rows) and population (columns). Notice that each row sums to a subtotal number of countries in that GDP range, and each column sums to a subtotal number of countries in that population range. Both the subtotal row and subtotal column sum to 50, the grand total. Moreover, notice that both population and GDP are shown in descending order: high population/high GDP countries are in the upper left corner of the table, while low population/low GDP countries are in the lower right corner of the table.
GDP between $10 billion and $20 billion is a single row in the table. Population between 10–50 million people includes two columns in the table. Look at the intersections between this row and two columns. There are three countries with populations of 10–20 million and GDPs of $10 billion to $20 billion. There are also three countries with populations of 20–50 million and GDPs of $10 billion to $20 billion GDP, for a total of six countries