Grid arithmetic

How this works

Each question shows a grid of numbers and one instruction. Find the numbers the instruction names, work out the answer and type it. After each answer the numbers that matter are marked in the grid, with the working under it.

A prime number has exactly two factors, 1 and itself, so 1 is not prime. Greater than 20 does not include 20. The top row is the first row, and the middle row has the same number of rows above and below it.

Level 1 is a 3 by 3 grid of numbers from 1 to 20. Each level adds cells, larger numbers, or instructions with two steps. Level 6 is a 4 by 5 grid of numbers up to 150.

Type a whole number. Spaces and commas between digits are ignored, so 1,350 and 1350 are the same answer.

An untimed round has no clock. A 3-minute round gives three minutes of answering for all eight questions. The clock stops while you read each answer, and questions you do not reach count as wrong.

Your score is how many of the eight you answered correctly. It is practice at reading an instruction carefully, not a measure of ability. Finish the round to save it in this browser. A timed round whose clock runs out is saved, as long as you answered one question. Ending a round early saves nothing.

Type the answer. Enter checks it, and Enter again moves to the next question. Escape ends the round. End ends it too, except while the caret is in the answer box.