On the cube made from this net, the two numbers on every pair of opposite faces have the same total. Which number belongs in the square marked ?

In the row 2 ? 10 the end squares fold opposite each other, so 2 is opposite 10: total 12. 7 and 5, sticking out on opposite sides of 2, are also opposite: total 12. That leaves 9 opposite ?, so ? $=12-9=3$. (5 comes from pairing ? with 7.)