Invariant
75 problems
Compute one expression several ways
Simplify long add-subtract chains
Fast sums using pattern and symmetry
Flipped digital clock readings
Clock seen in a mirror
Perimeter of a figure built from squares
Rectangle side and perimeter from equal squares
Which glyphs survive a turn or flip
Spot rotated copies of a given shape
Draw the figure a mirror doubles
Sum of every digit written
Find angles by grouping equal pairs
Angles from overlapping right-triangle set squares
Chase angles through triangles in a crossing figure
Angle made by folding rectangular paper
Sum of a polygon's exterior angles
Times whose clock-hand angle is acute
Where the hour hand sits within the hour
Surplus and shortage when sharing equally
One interior angle of a regular polygon
Cross every bridge exactly once
Draw a figure in one stroke
Move first or second in a blocking game
Which marbles you must take to win
Mirror the opponent to keep piles equal
Equal sides force equal base angles
Perimeter after cutting equilateral corners
Find the hidden isosceles triangle
Draw an auxiliary parallel through the bend
Chase angles using a rectangle's parallel sides
Prove triangle angle sum with a parallel line
Angles of a star built from rhombuses
Equilateral triangle inside a regular pentagon
Why a parallelogram's opposite angles are equal
Folding a rectangle makes an isosceles triangle
Angles where overlapped figures share a vertex
Angle where a rotated figure meets the original
Simplify calculations by regrouping
Perimeter of joined and cut rectangles
Area of overlapping figures
Area of pieces that split a rectangle
Move a figure to make its area easy
Equal-area triangles between parallel sides
Straighten a border without changing areas
Use odd and even to rule out totals
Lockers left open by divisor-count parity
The Euclidean algorithm for the GCD
Product equals GCD times LCM
Angles between segments drawn on a cube
Cut and fold edges of a net
Two-coloring decides domino tiling
Decide odd or even without computing
Flip count parity pins down hidden faces
Move the mean by adding a value
Solve using the sum of one line
Move a triangle onto its congruent twin
Figures turned through 180 degrees
Slide a face to an equivalent net
Rolling a die along a path
Simplify complex fractions
Simplify messy fraction-decimal expressions
Surface area of a composite solid
Read the time on a numberless clock
Euler's formula for polyhedra
Edges of a solid from its net
Dual polyhedron from face centers
Descartes' angle-defect theorem
Concentration of a salt solution
Unbracket, reorder, and factor out
Collapse a repeated number or expression
Central angle of a cone net
Parity bounds the fewest three-coin flips
Simplify messy decimal computations
Surface area unchanged by removing a cube
Area of the shaded part (2)