An easy question on number puzzles for P1–P2, with a step-by-step solution, diagrams and the answer. Number puzzles and made-up operations appear in the Hua Xia Cup from the P1 and P2 first rounds to the P6 finals.
The question
Ink has blotted Ming's maths homework: two digits in this sum can't be read.
Each □ hides one digit.
2
+9
40
What do the two hidden digits add up to?
Show the hints (2)
💡Hint 1: Start with the ones: the top □ plus 9 must end in 0. What is the □?
💡Hint 2: Do the ones carry? Then the tens: 2 plus the bottom □ plus the carried 1 must make 4.
Worked solution
Have a go first, then read the solution step by step.
1.Ones: 0 is less than 9, so it must be 10 − 9 = 1. The top □ is 1 (1 + 9 = 10), carrying 1 to the tens.
2.Tens: 4 − 2 − 1 = 1. The bottom □ is 1.
3.The two hidden digits: 1 + 1 = 2.
⚠️Common mistake: forgetting the 1 carried from the ones, and getting 2 for the bottom □.
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