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So let the no. be added be x
so the according to question 

(7 + x)/(5 + x) = 5/4
28 + 4x = 25 + 5x 
x = 3

so if we add 3 to both terms we get the required answer

ie (7+3)/(5+3) = 10/8 = 5/4
Let the number be x
then 28=4x=25+5x
⇒ x=3
The number is 3