Step by step
The question
Question: solve 3x + 5 = 20.
We want x by itself.
Two things are stuck to x: times 3 and + 5.
First x was times 3.
Then + 5.
To take them off, we go backwards.
First we remove the + 5.
Then the times 3.
There are four choices.
First we solve it, the normal way.
Then we pick.
1 Remove the + 5
To remove + 5, we take away 5.
We do it on both sides.
Why both sides?
The = sign means both sides are the same amount.
If we change only one side, they are not the same any more.
+ 5 and − 5 make zero.
Cross them out.
20 − 5 = 15.
Now we have 3x = 15.
2 Remove the 3
3x means 3 times x.
We want just x.
To remove times 3, we divide by 3.
We do it on both sides, so they stay equal.
3 divided by 3 is 1.
So just x is left.
15 divided by 3 is 5.
So x = 5!
3 Check
Let's check: put 5 in place of x.
We get 20, the same as the right side.
So x = 5 is right!
4 Pick the choice
Our answer is x = 5.
That is choice A.
5 Why not the others
B is 25/3.
Wrong: added 5: 3x = 25.
C is 15.
Wrong: 3x = 15, but we still had to divide by 3.
D is 45.
Wrong: 15 × 3: multiplied instead of dividing.
Let's look back.
First: remove the + 5.
Then: remove the 3.
Then: check.
Then: pick the choice.
Last: why not the others.