If they didn't tell you the total number of people in the class (only the percentages of each type; left and right handed) then you cannot determine how many of each there are. There could be any number since the class could be any size.
And did they say "being left handed is independent of gender"? Please explain the table you were given more clearly. As you have explained it appears that you have 3 columns and 3 rows of data, but only 4 categories (left-girl, right-girl, left-boy, right-boy).