Start preparation from zero point in concept layer and proceed to improve your ability to solve problems step by step. Never start preparation randomly from a higher level concept layer. This is the crux of the whole problem, as high speed problem solving in competitive tests heavily relies on total control on the basic concepts, be it maths, reasoning or English.
Break up a number into its factors, just as you break up a complex problem into its component simpler sub-problems. Factorization is one of the most basic operations and is used extensively in other more important problem areas of HCF and LCM...
Numbers form the very foundation of Mathematics whereas zero and place value mechanism together make it possible to form any number, however large or small it may be. After formation of numbers, the four basic arithmetic operations addition, subtraction, multiplication and division transform the numbers using again place value system as a base mechanism. On this foundation Mathematics grows further..