Add or subtract unlike fractions by crossing the diagonals — no LCM hunt
Butterfly — add or subtract fractions with unlike denominators by cross-multiplying, not by finding a common denominator.
For 3/4 + 2/5: cross-multiply the diagonals 3×5 = 15 and 2×4 = 8; add them for the top → 23; multiply the denominators for the bottom → 4×5 = 20. So the answer is 23/20. You enter the method step by step — the two cross-products, the top, then the bottom — and each step unlocks only when the one before it is right. Five levels climb from small additions to bigger denominators and subtraction; the steps fade away as you climb until only a bare fraction box is left, and a short timed boss caps each level.