Answer:
12,662 books
Step-by-step explanation:
Total number of books = 75,972
No. of science books = ⅓ of 75,972 = ⅓*75,972 = 25,324 books.
No. of business books = 75% of the remaining books = 75% of (75,972 - 25,324)
= 
No. of business books = 37,986
No. of maths books in the math section = 75,972 - (25,324 + 37,986)
= 75,972 - 63,310
= 12,662 books
7 + 28n + 4
28n + (7 + 4)
28n + 11
Solution :
2a + 2b = 7 ...1)
4a + 3b = 12 ...2)
In equation, 1)
a = (7 - 2b)/2 ...3)
Putting value of a in equation 2) we get :
4 × (7 - 2b)/2 + 3b = 12
2( 7 - 2b ) + 3b = 12
14 - 4b + 3b = 12
b = 2
Putting value of b in 3) we get :
a = ( 7 - 2×2)/2
a = 3/2 = 1.5
Now,
2x - 3y = 16 ...5)
x + 2y = -6 ...6)
x = -6 - 2y
Putting above value of x in eq 5) , we get :
2( -6 - 2y ) - 3y = 16
-12 - 4y - 3y = 16
7y = -28
y = -4
x = -6 - ( 2× -4 )
x = 2
Hence, this is the required solution.
Money earned by restaurant on Friday=$1073
Money earned by restaurant on Saturday=$1108
Let Money earned by restaurant on Sunday=$x
Average=$1000

Number of observation=3
⇒ 3× 1000=1073+1108+x
⇒3000= 2 181+x
⇒3000-2181=x
⇒x=819
The restaurant needs to earn on Sunday to average $1000 per day over the three-day period=$819