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177 problems, every one written from scratch, every wrong answer carrying the mistake that produces it. 708 explanations in all.

Level

1–2
3–4
5–6
7–8
9–10
11–12

Point value

3 pt
4 pt
5 pt

Strand

NUMNumber & Arithmetic
CNTCounting
GEOGeometry & Visualization
LOGLogic & Deduction
PATPatterns & Sequences
MODModeling & Word Problems
CFTContest Craft

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84 problemsClear all filters

KA-0033
4 points, difficulty 2 of 3Level 5–6not yet calibrated
The mean of five numbers is 12. One of the numbers is removed and the mean of the remaining four is 11. What number was removed?
NUM-15Means, and working backwards from a meanalso LOG-08
KA-0034
5 points, difficulty 3 of 3Level 5–6has a figurenot yet calibrated
A 4 by 4 board has two opposite corner squares removed, leaving 14 squares. Each domino covers exactly two squares that share an edge. Can 7 dominoes cover the board?
LOG-07Coloring argumentsalso LOG-05, GEO-14
KA-0035
3 points, difficulty 1 of 3Level 5–6not yet calibrated
Eight people meet and everyone shakes hands with everyone else exactly once. How many handshakes take place?
CNT-08Handshakes and pairsalso CNT-06
KA-0036
4 points, difficulty 2 of 3Level 5–6not yet calibrated
Ravi cycles 12 km at 24 km per hour, then walks 6 km at 4 km per hour. How many minutes does the whole journey take?
MOD-03Rate, time, distancealso MOD-07
KA-0037
5 points, difficulty 3 of 3Level 5–6not yet calibrated
What is the units digit of 7 multiplied by itself 2026 times, that is 7 to the power 2026?
NUM-07Last-digit behavior of products and powersalso NUM-06, PAT-03
KA-0038
3 points, difficulty 1 of 3Level 5–6has a figurenot yet calibrated
A solid block is built from unit cubes and measures 4 by 3 by 2. How many unit cubes does it contain?
GEO-13Building and counting with unit cubesalso CNT-14
KA-0039
4 points, difficulty 2 of 3Level 5–6has a figurenot yet calibrated
A flag shape is rotated a quarter turn clockwise about its bottom-left corner. Which of the descriptions matches the result?
GEO-09Reflections, rotations, translationsalso GEO-08
KA-0040
5 points, difficulty 3 of 3Level 5–6not yet calibrated
A drawer holds 10 red socks, 10 blue socks and 10 green socks, all mixed up in the dark. How many socks must you take out to be sure of having a matching pair?
CNT-17The pigeonhole principlealso CNT-18, LOG-09
KA-0041
3 points, difficulty 1 of 3Level 5–6not yet calibrated
One bus leaves every 12 minutes and another every 18 minutes. They leave together at 9:00. When do they next leave together?
NUM-05Factors, multiples, LCM and GCD by listingalso NUM-06
KA-0042
4 points, difficulty 2 of 3Level 5–6not yet calibrated
A photograph is enlarged so that each side becomes 3 times as long. How many times bigger is its area?
GEO-16Scaling and similar figuresalso GEO-01, NUM-11
KA-0043
5 points, difficulty 3 of 3Level 5–6not yet calibrated
A bag holds 5 black and 6 white stones. You repeatedly remove two stones: if they match you put a black one in, if they differ you put a white one in. What colour is the last stone?
LOG-06Invariants and monovariantsalso LOG-05, NUM-03
KA-0044
4 points, difficulty 2 of 3Level 5–6not yet calibrated
One tap fills a tank in 6 hours and another fills it in 3 hours. With both running, how long does the tank take to fill?
MOD-04Combined work ratesalso MOD-03, NUM-09

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