I converted certain rows/columns into equations to help me find out the values.
From the second row, where the total is 30, the equation is 2 hexagons + 2 squares = 30, therefore 1 hexagon + 1 square = 15.
So, in the 3rd column where the total is 23, the equation is 1 hexagon + 1 square + 2 circles = 23. We know that 1 hexagon + 1 square = 15 (from above) so 23 – 15 = 8 and if we divide 8 by 2 to get one circle, one circle equals 4.
After this, on the second column the equation is 3 circles + 1 triangle = 18. We know that one circle is 4 so when we times that by 3 we get 12. 18 – 12 = 6. This is what the triangle is worth.
If we now go to the 4th row, the equation for this is 3 circles + 1 square = 20. We already know that 3 circles are 12 so 20 – 12 = 8, this is what the square is worth.
If we now go back to the very first equation that was 1 hexagon + 1 square = 15, we can now solve this. If one square is 8, then 15 – 8 = the hexagon’s value, 7.
We are here so far:
Square = 8
Triangle = 6
Hexagon = 7
Circle = 4
Now we can work out the number of ?. The equation for this is 1 triangle +1 hexagon + 2 circles = ?. This can now be work out. Using the key above, we can work out that the equation means 6+7+8=? And if you work that sum out then the ? = 21. That is the answer: ?=21