Let the number be x in all those expressions.
15 - x = 20 - 7
or, x = 15 - 13 = 2
x + 2 = 16 + 3
or, x = 19 - 2 = 17
13 - 6 = x - 9
or, 7 + 9 = 16 = x
10 • 6 = 15 • x
or, 60/15 = 4 = x
10 ÷ x = 20 ÷ 4
or, 5 × 10 = 50 = x
18 • x = 16 • 9
or, x = 144 / 18 = 8
16 • 9 = 18 • x
or, x = 144 ÷ 18 = 8
10 ÷ 2 = x ÷ 2
or, x = 10 × 2 = 20