A phone has 80 percent and loses 5 percent per hour. Write the…

Step by step, with the reason for every step · Equation of a line from a table or a story · Algebra 1

Step by step

Question: A phone has 80 percent and loses 5 percent per hour.

Write the equation.

The percent left depends on the number of hours.

So x is the hours: the input.

y is the percent left: the output.

x = hours, y = percent left

Rate of change means: how much y changes when x goes up by 1.

Each hour, it goes down by 5.

So m is −5.

= m = −5

Careful!

It loses 5 each time.

Going down means a minus.

Careful: going down: m is negative

The start value is y when x is 0.

At the start, with 0 hours, it is 80.

= b = 80

A line is y = m x plus b: m is the rate of change, b is the start value.

y = mx + b

So the equation is y = −5x + 80.

= y = −5x + 80

Let's check with 4 hours: the equation gives 60.

In the story: 80, minus 5, minus 5, minus 5, minus 5: 60.

So the answer is right!

x = 4: −5 × 4 + 80 = 60

Let's look back.

First: which depends on which.

Then: rate of change.

Then: start value.

Then: write the equation.

Last: check.

Answer

= y = −5x + 80

Try this one yourself, then check:

Show the answer

y = −5x + 79