One grid, six equations.

A 3×3 grid of numbers with operations between them. Reading across, each of the top two rows is an equation whose result sits in the third column. Reading down, each column is an equation whose result sits in the bottom row:

 a  +  b  =  c
 +     −     +
 d  +  e  =  f
 =     =     =
 g     h     i

Every row and column must hold simultaneously — six equations sharing nine cells.

Some cells are given; fill in the blanks so all six equations are true. Early grids use only addition; harder ones mix in subtraction and reveal fewer givens.

You and your opponent get the same sequence of grids on a 90-second clock. Solve a grid and the next one appears, a bit harder. Most grids completed when time runs out wins — ties break on accuracy, then speed.

Don’t fill cells left to right. Find the row or column with the most givens — with two of its three cells known, the third is forced. Each forced cell makes another line solvable, and the grid unzips.