How can I used rational equations in 3 real life examples

The final cost of a sale is determined by multiplying the price on the tag by 75%. which best describes the function that represents the situation?

Asystem of equations has 1 solution.if 4x-y=5 is one of the equations , which could be the other equation ?

You are standing next to a really big circular lake. you want to measure the diameter of the lake, but you don't want to have to swim across with a measuring tape! you decide to walk around the perimeter of the lake and measure its circumference, and find that it's 400\pi\text{ m}400? m. what is the diameter dd of the lake?