If you drive a car or have ever flown in an airplane, you've probably noticed that time, speed, and distance are related. Here's the basic formula for distance (d), which equals speed (called velocity in science and represented by v) multiplied by time (t):


From this simple formula, you can derive these other formulas as well:


By knowing any two of the components, you can use these formulas to figure out the third. If you know how far you've traveled and the time the journey has taken, you can calculate your average speed. If you know the distance and the average speed, you can calculate the time you've been driving.

This "know two to get all three" trick applies to many day-to-day math activities: buying lumber (length needed/price per board foot/total cost), buying cases of motor oil (price per can/number of cans in a case/total cost), or buying meat at the grocery store (weight of cut/price per pound/total cost).