Sizu Home › Calculators › Find the Missing Number
Opens on Chapter 1’s example: Roz’s first equation: a number times 3, plus 5, makes 20.
A number, times a, plus b, makes c
Enter your numbers, then press Work it out.
How it works
The puzzle a × (number) + b = c was built in two steps: multiply, then add. Working backward undoes them in reverse order: first take away b (or add it back if it was taken away), then divide by a. Putting the answer back into the puzzle is the check: if both sides agree, it’s right. Guess and check gets there too, but slowly when the answer isn’t a whole number.
From the book, Chapter 1. Roz’s 3x + 5 = 20: take away 5 to get 15, divide by 3 to get 5. Doubled plus 4 makes 26 gives 11; 4n + 9 = 61 gives 13 (a guess of 10 makes 49, too low; 15 makes 69, too high); the bus pass is 85 a month, and $20 less $6.35 change means $13.65 was spent. Every chapter and its calculator →
Numbers can be typed as 1,450, $38, 0.40, 12%, 1/6 or 2 3/8. The algebra is done with exact fractions, so 1/3 stays 1/3; decimals are rounded half up, the way the book rounds. Everything stays in your browser: no sign-up, nothing stored or sent.