There are 42 students in a class.
Each student walks to school or cycles to school or gets the bus to school.
There are 23 girls in the class.
10 of the girls walk to school.
6 of the boys cycle to school.
7 of the 11 students who get the bus to school are boys.
Find the number of these students who walk to school.
Your final line must say, Number who walk to school = ...