Comment by afiori

> but how do you know that is a number?

Quick answer: math[0]

Slightly longer answer decimal numbers between 0 and 1 can be written as the sum of a_0*10^0 + a_1*10^1 + a_2*10^2 + ... + a_i*10^i + ... where a_i is one of 0,1,2,3,4,5,6,7,8,9. for series in this shape you can prove that the sum of two series is the same iff and only if the sequence of digits are all the same (up to the slight complication of 0.09999999 = 0.1 and similar)

[0] https://en.wikipedia.org/wiki/Series_(mathematics)