Each square has a value: in its row, it is worth its column number (shown across the top); in its column, it is worth its row number (shown down the left).
Shade squares so that every row adds up to the number on its right and every column adds up to the number underneath. A row total of 1 means only the first square; a total equal to 1+2+…+N means every square.
The trick is listing the ways to make a total: in a 5-wide row, 6 can be 1+5, 2+4 or 1+2+3. Combine that with the columns and only one set survives.
Shade squares so each row and column adds up to its total — a shaded square is worth its column number in its row and its row number in its column.
The full guide, with a worked example and common mistakes: How to play Kakurasu.
Across the top: what a shaded square is worth in its row. Down the left: what it is worth in its column. Totals are on the right and at the bottom.
Adding small numbers and finding all the ways to make a total — the same skill as number bonds and partitions.
Yes — every puzzle here is checked to have exactly one solution.
Open the Kakurasu Solver, type the totals and press Solve.