mental calculation

the looks on the faces of the students in this 1895 painting by n. bogdanov-belsky are wonderful. i love moments when everyone is really focused on trying to figure something out. they are counting in their heads and feeling hopeful and confident that they can solve the puzzle shown on the board if they just think a little more...


try it - no calculating aids allowed!


via dark roasted blend

ps. the answer is discussed in the comments, so have a go at solving it before you peak!