Question 455711
<pre>
Time (min) | 5 | 10 | 15 | 20 | 25 | 
Charge ($) | 8 | 13 | 18 | 23 | 28 |

I'll show you three ways to find it.

Plot the 5 points on a graph:

{{{drawing(600,600,-5,30,-5,30,
circle(5,8,.2),
circle(10,13,.2),
circle(15,18,.2),
circle(20,23,.2),
circle(25,28,.2),
circle(5,8,.15),
circle(10,13,.15),
circle(15,18,.15),
circle(20,23,.15),
circle(25,28,.15),
graph(600,600,-5,30,-5,30) )}}}

Draw a line through them

{{{drawing(600,600,-5,30,-5,30,
circle(5,8,.2),
circle(10,13,.2),
circle(15,18,.2),
circle(20,23,.2),
circle(25,28,.2),
circle(5,8,.15),
circle(10,13,.15),
circle(15,18,.15),
circle(20,23,.15),
circle(25,28,.15),
green(line(-7,-4,40,43)),
graph(600,600,-5,30,-5,30) )}}}

Now you can see that the green line crosses the y-axis at 3,

That means the y-intercept is the point (0,3).

Another way to do it is to look at your list:

Time (min) | 5 | 10 | 15 | 20 | 25 | 
Charge ($) | 8 | 13 | 18 | 23 | 28 |

Notice that the times are spaced out in steps of 5 minutes
starting with 5

Also notice that the charges are spaced out is steps of 5,
starting with 8.

Now suppose we wanted to put in a point before (5,8). It 
would have to be <font color = "green">(0,3)</font>

Time (min) | <font color = "green">0</font> | 5 | 10 | 15 | 20 | 25 | 
Charge.($) | <font color = "green">3</font> | 8 | 13 | 18 | 23 | 28 |

That's because the point before the first one would have to be
gotten by subtracting 5 from 5 getting 0 and 5 from 8 getting 3.
So the y-intercept is <font color = "green">(0,3)</font>

You can also do it by first finding the slope using the first two
points, (5,8) and (10,13) and then using the point-slope form.

{{{m=(y[2]-y[1])/(x[2]-x[1]) = (13-8)/(10-5) = 5/5 = 1}}}

{{{y-y[1]=m(x-x[1])}}}

{{{y-8=1(x-5)}}}

{{{y-8=x-5}}}

{{{y=x+3)}}}

Now the y-intercept is gotten by substituting x=0

{{{y=0+3)}}}

{{{y=3)}}}

So the y-intercept is (0,3).

Edwin</pre>