Answer:
Below.
Step-by-step explanation:
To find the number of students in each grade you multiply the marks by the frequency density. The number of students is given by the area under the rectangles.
So to find the number of students with grade U (< 160) it is:
120 * 0.2 + 40*0.5
= 24 + 20
= 44.
The number for Grade E
= (200 - 160) * 0.5
= 40 * 0.5
= 20.
The others can be found in the same way.
Step-by-step explanation:

Let a, b, and c be the times each pump will fill the tank when working alone.
Therefore, in 1 hour;
1/a +1/b = 1/(6/5) = 5/6 ---- (1)
1/a+1/c = 1/(3/2) = 2/3 ---- (2)
1/b+1/c = 1/(2) = 1/2 ---- (3)
From equation (1)
1/a = 5/6-1/b
Substituting for 1/a in eqn (2)
5/6-1/b+1/c = 2/3
-1/b +1/c = -1/6 => 1/c = 1/b - 1/6 --- (4)
Using eqn (4) in eqn (3)
1/b+1/b-1/6 = 1/2
2/b-1/6 = 1/2
2/b =1/2+1/6 = 2/3
1/b = 1/3
Then,
1/c = 1/3 - 1/6 = 1/6
1/a = 5/6 - 1/3 = 1/2
This means, in 1 hour and with all the pumps working together, the tank will be filled to;
1/a+1/b+1/c = 1/2+1/3+1/6 = 1 (filled fully).
Therefore, it will take 1 hour to fill the tank when all pumps are working together.
Answer:
Elimination is the best to solve it