Question 108942
one third of the class doesont finish the test on time 
beacause

the whole population of the class is 3/3
and 2/3 completed the homework on time
therefore...............
suppose the students who haven't done the homework is"x"

x + 2/3 = 3/3
x = 3/3 - 2/3
x = 3-2/3
x = 1/3     ANS