Step by step
The question
Question: for which k do x + 2y = 5 and 3x + ky = 15 have infinitely many solutions?
We call them ① and ②.
Infinitely many solutions means every point on a line works for both.
1 Make the x parts the same
To compare the equations, we make the parts of one letter the same.
k is with y, so we use the other letter, x: it has plain numbers in front.
The x parts: x in ① and 3x in ②.
3 divided by 1 is 3.
So x times 3 is 3x.
We multiply every part of ① by 3.
We do it on both sides, so they stay equal.
3 times x is 3x.
3 times 2y is 6y.
3 times 5 is 15.
We call it ③.
2 Compare ③ and ②
We write ② again, under ③.
If the left sides are the same, both equations talk about the same amount, 3x + 6y.
One amount cannot be two different numbers at once.
So no point works: no solution.
If the numbers are the same too, it is one equation written twice.
Every point on that line works: infinitely many solutions.
If the left sides are different, we can take one equation away from the other.
Some y is left, so we can find y.
That gives one solution.
Careful!
We compare with ③, not ①.
① was times 3.
2y times 3 is 6y.
3 Find k
We want infinitely many solutions.
So the left sides must be the same: ky = 6y.
k times y is 6 times y when k is 6.
So k = 6.
And the numbers, 15 and 15, are the same too.
Good.
4 Check
Let's check.
We put k = 6 into ②.
Take ② away from ③.
The left sides are the same, so together they make 0.
The numbers: 15 take away 15.
0 = 0 is always true.
They are the same equation.
It works!
On a grid, both equations give the same line.
② lies right on top of ①.
Every point on the line works for both: infinitely many solutions.
Let's look back.
First: make the x parts the same.
Then: compare ③ and ②.
Then: find k.
Last: check.