x is the independent variable, and in this scenario, the independent variable is the number of days overdue. So the best equation would be where x is the number of days overdue, and y is the cost in cents.
Example equation:
x = number of days overdue
y = cost in cents
z = total price
z = yx
The cat prowled and the mice scattered
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Let, the speed of boat = x
Speed of current = y
So, x + y = 280/7
x + y = 40 ---- first equation
x - y = 280/14
x - y = 20
x = 20 + y
Substitute this value of x in first equation,
20 + y + y = 40
2y = 40 - 20
y = 20/2
y = 10
Substitute this in first equation,
x + 10 = 40
x = 30
In short, Speed of Boat = 30 mph, & speed of current = 10 mph
Hope this helps!
