A dependent variable is a variable whose value will always depend on the value of the independent variable.
For example, suppose it costs "10 dollars per pound" to buy lobster.
The cost or the amount that you pay is a dependent variable because it depends on the number of pounds of lobster that you buy.
1. A recipe says to use 1/2 tablespoon of salt for every 2 pounds of ground meat.
The number of tablespoons of salt that you use is a dependent variable since the amount depends of how much ground meat you use.
2. A car can travel about 30 miles on one gallon of gas.
The number of miles you can travel is a dependent variable since the distance you can travel depends on how much gas you have in the vehicle.
3. Let R = 2X + 1 be the revenue that you generate for selling an item with X being the number of items you sell.
R or the revenue is a dependent variable since your revenue depends on the number of items you sell.