Question

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question 12

Answer 1

Let the number to be added be x.
Then,
5+x:9+x::7+x:12+x
5+x/9+x=7+x/12+x
By cross multiplication
(7+x)(5+x)=(9+x)(12+x)
35+12x+x2=108+21x+x2
12x-21x=108-35
-9x=63
x=63/-9
x=-7

Answer 2

let the no. be x.
(5+x):(9+x): : (7+x):(12+x)
5+x/9+x=7+x/12+x
(5+x)(12+x)=(9+x)(7+x)
5(12+x)+x(12+x)=9(7+x)+x(7+x)
60+5x+12x+x^2=63+9x+7x+x^2
60+17x+x^2=63+16x+x^2
17x-16x+x^2-x^2=63
x=63