Question 634455
L = {x l x is an integer between 20 and 40, inclusive}


L = {20, 21, 22, 23, ... , 35, 36, 37, 38, 39, 40}


So we're trying to count all the integers from 20 to 40 (including both 20 and 40). So we can use the handy shortcut


Number of numbers from X to Y = Y - X + 1


Number of numbers from 20 to 40 = 40 - 20 + 1


Number of numbers from 20 to 40 = <font color="red">21</font>


So the cardinality of set L is <font color="red">21</font>