A more formal definition for a number less than 20 could be, "a number x, x < 20" or if you want to express the set of all numbers less than 20 "{x | x < 20}" which is read "the set of all numbers x such that x is less than 20". In interval notation, you can write (-infinity, 20) where "infinity" should be replaced by the infinity symbol.
Let the number be x and so the expression would be: x+20
20 x (1 + (15/100)) = 20 x 1.15 = 23
10 x 20 = 200
6 x 3 + 2 = 20
54 x (1 - 20/100) = 43.2
(x/20)+9 where x is the number
4+(x/100)5=x/20+4
50 x 1.20 = 60
20 x 4 10 x 4 x 2
(20 + x)-3
It's 20 then. 4 x 4 = 16 5 x 5 = 25 20 is less than 25 and 20 is also more than 16.