Step by step
The question
Question: y varies directly with x.
y = 21 when x = 7.
Find y when x = 10.
1 Write the rule
Varies directly means y is always the same number times x.
When x gets bigger, y gets bigger too.
We call the number k: y = kx.
k stays the same for every pair of numbers.
Books call it the constant of variation.
2 Find k
We put in y = 21 and x = 7.
k times 7 is written 7k.
7k means 7 times k.
We want just k.
To remove times 7, we divide by 7.
We do it on both sides, so they stay equal.
Careful!
k is y divided by x: 21/7.
Not 7/21.
The y number goes on top.
21/7 on the left.
7k/7 on the right.
7 divided by 7 is 1.
So just k is left.
21 divided by 7 is 3.
So k = 3.
3 Write the equation
We put 3 in place of k.
So the equation is y = 3x.
4 Find y
We put in x = 10.
3 times 10 is 30.
So y = 30.
5 Check
Let's check.
In direct variation, y divided by x is always k.
First pair: 21/7 is 3.
New pair: 30/10 is 3 too.
The same k.
So the answer is right!
Let's look back.
First: write the rule.
Then: find k.
Then: write the equation.
Then: find y.
Last: check.