Persistent Systems Interview Question

1 st question I got was based on aptitude.First question boils down to solving for three variables using 2 equations.Second question was to write a program to print no of occourances of all characters in a string.Third question was on 4 fundamentals of oops.fourth question was on operating systems.it asks whether we can run a program whose size is greater than size of RAM.and the last question was on software development life cycle.