The generic equation of the line is:
y-yo = m (x-xo)
The slope is:
m = (y2-y1) / (x2-x1)
We choose ordered pairs that are closest to the line shown:
m = (20-10) / (6-3)
m = 10/3
(xo, yo) = (3, 10)
Substituting:
y-10 = (10/3) (x-3)
y = (10/3) x - 10 + 10
y = (10/3) x
For y = 10 we have:
40 = (10/3) x
x = (3/10) (40)
x = 12
Answer:
the age of a child who has visited the doctor 40 times is:
12 years old
