Click in cell D3 and subtract the Purchase amount from the Sales amount. To do this, enter an equal sign (=) in cell D3, click in cell C3, enter a minus sign (–) and then click in cell B3 (see Figure 1-18).

Figure 1-18. Subtracting values.

Press the Enter key.

Double-click the fill handle to calculate the profit margin through December (see Figure 1-19).

Figure 1-19. The profit margin is calculated for all months.