In my School......

Algebra Level 4

In a school of student strength 500,two-thirds of the students,who do not wear spectacles do not bring lunch.three-quarters of the students,who do not bring lunch,do not wear spectacles.Altogether 60 spectacles students bring lunch too. Find the number of students who do not bring lunch and do not wear spectacles.

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The answer is 240.

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3 solutions

Ansh Bhatt
May 8, 2015

take the number of students who wear spectacles as 'x'

.'. x - 60 students wear spectacles and don't bring lunch........ eq. 1

.'. {2(500 - x)}/3 students don't wear spectacles and don't bring lunch........... eq. 2

eq 1 is three times eq. 2 [eq. 1 equals 1/4 of students who don't bring lunch]

.'. 3x - 180 = {2(500 _ x)}/3

.'. 9x - 540 = 1000 - 2x

.'. 11x = 1540

.'. x = 140

substituting value of x in eq. 2 we get,

360 * 2/3 = 240

Let N S NS be the set of students who do not wear spectacles and N L NL be the set of students who do not bring lunch. The set of students who wear spectacles and who bring lunch is the complement of the set that is the union of sets N S NS and N L , NL, and thus this union has 500 60 = 440 500 - 60 = 440 students.

Letting A |A| represent the number of elements in any set A , A, we then have that

N S N L = N S + N L N S N L = 440 , |NS \cup NL| = |NS| + |NL| - |NS \cap NL| = 440, (i).

We are given that N S N L = 2 3 N S = 3 4 N L N L = 8 9 N S . |NS \cap NL| = \dfrac{2}{3}|NS| = \dfrac{3}{4}|NL| \Longrightarrow |NL| = \dfrac{8}{9}|NS|.

Plugging these results into equation (i), we find that

N S + 8 9 N S 2 3 N S = 440 11 9 N S = 440 N S = 360. |NS| + \dfrac{8}{9}|NS| - \dfrac{2}{3}|NS| = 440 \dfrac{11}{9}|NS| = 440 \Longrightarrow |NS| = 360.

Thus the number of students who have neither spectacles nor a lunch is

N S N L = 2 3 N S = 2 3 360 = 240 . |NS \cap NL| = \dfrac{2}{3}|NS| = \dfrac{2}{3}*360 = \boxed{240}.

Beautiful way to use set theory. Thank you.

Niranjan Khanderia - 4 years, 9 months ago

The sketch above gives one solution and solution below is another solution.
From simple logic. S students with spectacles , L taking lunch.
NS not S............. NL not L.
(1-2/3) NS+60=L=500-NL ...... \implies NS + 3NL= 3 * 440.................(1)
(1-3/4)
NL+60=S=500-LS ...... \implies NL + 4NS= 4 * 440.................(2)
(1)+(2)...................................................4NL +5NS= 7 * 440.................(3)
4(2)-(3).........................................................11NS =9* 440.
NS=360.......NL=320
Those not hot having at lest one = 360 +320 = 680.
But there are only 500 - 60 = 440 such students left.
680 440 = 240. \implies\ 680 - 440 = \Large\ \ \ \color{#D61F06}{240}. are those not having spectacles nor lunch.



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