Math fun

 Actually he is not a math genius, WHY?

let me explain why,

when 1 is divided by 3 [1/3] the result is 0.3 bar [0.333333.....] the number three after decimal point keeps going without an end. so when we multiply this with 3 [ 0.333333... x 3] we get 0.9 bar [0.9999999999999...] so now the 9 after the decimal point never ends so the difference between this and one is [1 - 0.99999999999..] is 0.000000000000....0001 here the total number of digits after decimal point is determined by the number of digits after decimal point in 0.9999999...

so as it is infinite the number of zeros between the decimal point and the last number 1 is infinite, 

Which makes it a too small value, 

but we can prove that 0.9999999999... is equal to 1 by the following steps 

lets take x=0.9999999..

then 10x = 9.999999...

now 10x - x = 9.9999.. - 0.99999.. 
               9x = 9

so then x = 9/9 = 1

  we  therefore get  x = 1. 

So this means that 0.00000...000001 is a negligible value and we simply take the value 0.99999... as 1.

Or we can convert 0.3333... into  fraction by the following steps below:

if we take x = 0.3333..

10x = 3.3333...

10x - x = 3.3333.. - 0.33333...

9x = 3

x = 3/9

x = 1/3

therefore we get x = 1/3, when we substitute this value in the question he asked we get 

1/3 x 3 = 1. Very simple. 

this is a simple way to solve the question, so to stop wasting time in calculating problems like this i prefer to convert the value in fractional form unless they are given not to do so.