Target question => what percentage of the 20 students in Mr. Henderson's class are left-handed?
Total students = 20

Statement 1: Of the 12 girls in the class, 25% are left-handed.
Total girls = 12; left-handed girls = 25% * 12 = 3
Right-handed girls = 12 - 3 = 9
Total boys = 20 - 12 = 8
However, the number of left-handed boys is unknown. Therefore, statement is NOT SUFFICIENT.

Statement 2: 5 of the boys in the class are right-handed.
There is no information about left-handed boys and girls. So, statement 2 is NOT SUFFICIENT.

Combining both statements together:
From statement 1:
Total girls = 12; left-handed girls = 3
Right-handed girls = 9
Total boys = 8

From statement 2:
Right-handed boys = 5

Left-handed boys = 8 - 5 = 2
Total left-handed students = 3 + 3 = 6
% of students who are left-handed = (6/20) * 100 = 30%

Both statements combined together ARE SUFFICIENT.
Answer = option C