Explanation
CONCEPT:
The number of combinations of n things taken r at a time is given by nCr=r!(n−r)!n!.
CALCULATIONS:
Consider the first 30 natural numbers and the selection of two numbers 'a' and'b'could be selected as 30Cr.
As a2−b2 is divisible by 3
⇒(a+b)(a−b)=3p
If(a-b) is multiple of 3, then a2−b2 will be multiple of 3 also
These numbers (1,4,7,…….28),(2,5,……29),(3,6,9,….30)are possible:10C2+10C2+10C2=45×3=
135
If(a+b) is multiple of 3, then a2−b2 will be multiple of 3 also
∴These numbers {(1,2),(1,5),……(1,29)},{(4,2),(4,5)…}…(28,29)}
∴So total cases are=10×10
Total numbers are 135+100=235
∴Required probability=32530C2=30×25235×2=8747