Question 993340
Let teacher can grade t students per hour and assistant can grade s students per hour
In 4 hours, no. of students graded = 4t + 4s
In 5 hours assistant can grade alone
so no. of students as per him = 5s
so, 4t + 4s = 5s
=> 4t = s
So time taken by teacher alone= (4t + 4s)/t
= (4t + 16t)/t
=20t/t
=20 hours.