Question

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y is inversely proportional to x.
When y = 7, x = 9
a) Work out an equation connecting y and x.
b) Work out the value of y when x = 21.
How do i find k?

Answer 1

a) y = 63/x

b) y = 3

Explanation:

Inverse proportionality between two variables means that if one of them increases, then the other one decreases.

These type of relationships can be written as:

y = k/x

Now we know that when y =7, x = 9

Then we can replace these two values and find the value of k.

7 =k/9

7*9 = k

63 = k

The our relationship is:

y = 63/x

This is the solution to a).

Now let's go with b)

We want to find the value of y when x = 21. then we just need to replace x by 21 in the equation "y = 63/x"

We will get:

y = 63/21 = 3

When x = 21, y is equal to 3.