Question

the range of set of four number is 9 if three are18, 22, 21 what couldbe the fourth number

Answer 1

range-HV -LV
= 22-x=9
=  x=13

so the fourth no. is 13

Answer 2

range of a set is defined as the difference of highest value and lowest value in the set.
Let
22 be the highest value then the lowest value = 22 - 9 = 13
13 is the fourth number
Let
18 is the lowest number then the highest value = 18 +9 = 27
27 is the fourth number

therefore the fourth number can be 13 or 27