A simple example of a constant rate is a car with cruise control set at 60 mph. You know that in 10 minutes you have traveled 10 miles and that in 30 minutes you have traveled 30 miles. It means that if you stay at 60 mph and you know the distance left to your destination is 90 miles that you will arrive in 1 1/2 hours, because everything is constant. Slow down slightly and then increase your speed and the constants disappear. It's now very difficult to calculate your journey time with any accuracy.
An increasing constant rate in math is when numbers change, but always at the same rate; often referred to as the constant rate of change. For example, the following numbers are increasing at a constant rate: 1 x 2 = 2, 2 x 2 = 4 and 3 x 2 = 6. The multiplication factor of 2 remains constant, while the first number is increasing constantly at the same rate in each calculation, as is the result of the calculation. The same isn't true if the numbers were: 1 x 2 = 2, 5 x 2 = 10 and 7 x 2 = 14. While the factor 2 remains constant, there is no sequence to the first number or the result of the calculation, therefore the rate of increase cannot be described as constant.
The same rule applies to decreasing numbers which are effectively the reverse. In order for them to be defined as "constant," a sequential number pattern needs to be established. Assume you start with 100 apples in a box. Every 10 minutes someone buys 5 apples, so after 10 minutes you have 95 apples left, after 20 minutes 90 apples and so on. One constant is that it's always five apples each time and the other set constant is the time: 10 minutes. The constant rate of decrease for the number of apples left is 5. However, if the time between buying apples and the amount purchased each time varies, nothing can be described as constant.
The previous sections clearly show that for a rate to be considered constant in math there must always be a pattern. The numbers can increase, decrease or remain static, but they must be sequential. A constant rate enables you to undertake thousands of accurate calculations. For example, using the apples again, because you know how many apples are purchased every 10 minutes, you can calculate at what time exactly you will run out of apples so you can plan ahead.