Finding unknown numbers in equations
Maths · Key Stage 2 · The School
An equation with a gap
An equation like n add 7 equals 15 has a missing number hidden inside it. n stands for that missing number. Solving the equation means working out exactly what n must be, so that both sides of the equals sign genuinely balance. Think of it as a puzzle with one missing piece. Your job is to find the one number that makes it fit.
Undoing the operation
To solve n add 7 equals 15, undo the add 7 by doing the opposite: subtract 7 from both sides. 15 take away 7 is 8. So n equals 8. Whatever was done to the letter, the opposite operation, done to both sides, peels it away again. Addition and subtraction undo each other. So do multiplication and division, which is exactly the pair you use for a trickier equation.
Always check your answer
Put your answer back into the original equation to check it. If n equals 8, then n add 7 becomes 8 add 7, which is 15. That matches the right-hand side exactly, so the answer is confirmed correct. Checking takes seconds and catches almost every careless slip. Skipping this step is the single most common reason a correct method still ends in a wrong final answer.
Solving a word problem
A number, doubled, then with 3 added, makes 19. Write it as an equation: 2n add 3 equals 19. Undo the add 3 first: subtract 3 from both sides, leaving 2n equals 16. Undo the times 2 next: divide both sides by 2, leaving n equals 8. Check it: 2 times 8, add 3, makes 19. Correct.