The cost for 5 of the gift cards is $100.
The general equation of a straight line is,
y = mx + c
where, m is the gradient
and c is the intercept on y-axis.
Let x be the number of gifts cards and y is total cost (in $).
Now,
we will find m and c using the points (1,20) , (2,40)
The slope, m = (40-20)/(2-1)
m = 20
∵ Equation of straight line is given as,
y = mx + c
Put the value of m,
∴ y = 20x + c
Substitute with the point (1,20) ⇒ y = 20 at x = 1
∴ 20 = 20 * 1 + c
or,20 = 20 + c
∴ c = 0
∴ The equation of the line, y = 20x
To find the cost for 5 of the gift cards substitute with x = 5
∴ y = 20 * 5 = 100
So, the cost for 5 of the gift cards = $100
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