A monitor is 21 inches by 12 inches.  There are 100 dots per inch.  How many total pixels are there?
The display is 21*100=<T>multiply(21, 100)=2100 pixels across
The display is 12*100=<T>multiply(12, 100)=1200 pixels tall
That means there is a total of 2100*1200=<T>multiply(2100, 1200)=2520000 pixels
,520,000