How I worked it out:
column 2:
1 triangle + 3 squares = 30
and 2 triangles + 2 squares = 28
one of the squares has been replaced by a triangle so the total has gone down by 2.
This means that triangle + 2 = square
row 1:
if 2 triangles + 2 squares = 28
then 1 triangle + 1 square = 14
14 - 2 = 12
so 2 triangles = 12
12 / 2 = 6
so triangle = 6
triangle + 2 = square
so 6 + 2 = 8
and square = 8
row 4:
3 circles + square = 20
we know that square = 8
so 20 - 8 = 12
3 circles = 12
12 / 3 = 4
so circle = 4
row 2:
2 squares + 2 hexagons = 30
we know that square = 8
so 30 - (2 x 8) = 14
this means that 2 hexagons = 14
14 / 2 = 7
hexagon = 7
column 1:
triangle + hexagon + 2 circles = ?
we know that:
triangle = 6
hexagon = 7
circle = 4
6 + 7 + (2 x 4) = 21
so ? = 21
answers:
triangle = 6
square = 8
circle = 4
hexagon = 7
? = 21