Adding
15+15=
30
3 sides
triangle
2, 4, 6, 8, ?
10
20-10=
10
2x10=
20
25+25=
50
one side
circle
5, 10, 15, 20, ?
25
50-20=
30
4x10=
40
35+35=
70
4 equal sides
square
3, 6, 9, 12, ?
15
80-30=
50
7x10=
70
45+45=
90
2 short sides and 2 long sides
rectangle
150, 250, 350, 450, ?
550
90-50=
40
11x10=
110
55+55=
110
6 sides
hexagon
18, 22, 26, 30, ?
34
100-20=
80
18x10=
180