Foundation
What is substitution?
Substitution is the skill at the heart of Volt Racers. Every battery you evaluate, every equation you check — it all comes down to this one idea.
x is just a number in disguise
In algebra, x is a placeholder — a space reserved for a number that will be filled in later. When your car shows x = 4, that means: wherever you see x in an expression, replace it with 4.
That's all substitution is. Replace the letter with the number, then do the arithmetic.
- Replace x with 43 × 4 + 1
- Multiply first (before adding)12 + 1
- Add13 ✓
Key point
You always multiply before you add or subtract. This is the order of operations — and getting it wrong is the most common mistake at this level.
Multiply before you add
Look at the expression 3x + 1 with x = 4. There's a very common mistake here — adding before multiplying.
Wrong
4 + 1 = 5 first, then 3 × 5 = 15. This is incorrect — you added before multiplying.
Correct
3 × 4 = 12 first, then 12 + 1 = 13. Multiply first, then add.
The rule is: multiplication and division always happen before addition and subtraction. In an expression like 3x + 1, the "3x" part (3 times x) must be calculated before you deal with the "+ 1".
Try these with x = 5
- Replace x with 52 × 5 + 3
- Multiply10 + 3
- Add13 ✓
- Replace x with 54 × 5 − 7
- Multiply20 − 7
- Subtract13 ✓
- Replace x with 510 − 5
- Subtract5 ✓
No coefficient here — just replace and subtract directly.
The two mistakes everyone makes at first
Mistake 1
Keeping x as a letter
Some people write 3 × x + 1 and then stop, thinking the answer contains x.
Once you know x = 4, there is no x left. Replace every x with the number and finish the arithmetic.
Mistake 2
Adding before multiplying
In 3x + 1, doing the "+ 1" step before the "3x" step.
Multiplication always comes before addition. 3x means 3 times x — that multiplication must happen first.