Answer:
247
Step-by-step explanation:
Dependent variable is dependent on the independent variable
independent variable influences the dependentvariable
number of months influences the hcost
independent=number of months
dependent=cost
for the cost for 4 or 2 months, just start at months=2 or 4 and go up till you hit the line
equation=y=mx+b
m=slope
b=yintercept or starting point
solid line
initial fee=20
see attachment, black lines are what they are
2 months=$80
4months=140
rate is $30 per month
equation is c=30m+20
dashed line
intial fee=50
attachment, blue is dashed points
2months=$70
4months=$90
rate is $10 per month
equation is y=10x+50
Answer:
Option (B) is correct .
The constant of proportionality is 7.5 .
Reason
Let us assume that the number of books be x.
Let us assume that the cost of books be y .
Than

y = kx
Where k is constant of proportionality .
As given
Number of Books 4 6 8
Cost ($) 30 45 60
Thus
when x = 4 , y = 30
Put in the equation y = kx .


k = 7.5
when x = 6 , y = 45
Put in the equation y = kx .


k = 7.5
when x = 8 , y = 60
Put in the equation y = kx .


k = 7.5
Therefore the constant of proportionality is 7.5 i.e option (B) is correct .