Question: how to calculate percentage in aptitude test?
Answer: Percent implies “for every hundred”. ... 
To calculate p % of y. ... 
To find what percentage of x is y: y/x × 100.
To calculate percentage change in value. ... 
Percentage point change = Difference of two percentage figures.
Increase N by S % = N( 1+ S/100 )
Decrease N by S % = N (1 – S/100)