The ratio of male to female is 1:2, which means that for every one male, there are two females.
Let the number of males be x. Then, the number of females is 2x.
The total number of students is 42:
x + 2x = 42
3x = 42
x = 14
So, there are 14 males and 2x = 28 females in the class.
Therefore, there are 28 females in the class.