Exercises in this format use both advanced prefrontal circuits and — especially in their diagramless cousins — they also stimulate visuo-spatial rotation skills concentrated more in the right hemisphere. Work out these tasks without a calculator because keeping the alternative possibilities in mind sharpens Working Memory essential to logical thinking and planning how best to achieve set goals.

Addlocks are like crossword puzzles with interlocking numbers instead of words. Write a single digit in each box so the sum of the digits equals the total given in the clue for that row or column. No number is used more than once in any answer, and zero is not used.

In this addlock, all answers end with an odd digit.

Click instructions to find out more about how to play addlocks.

The active box and clue show green.

Wrong answers show red.

Right answers are black.