Question

_+_+_+_+80+90=100 what we will fill in the blank the answer will come 100. this is not decimal sum

Answer 1

It is not a riddle instead of a riddle it is a joke.
It is like that Akad Bakkad Bambe Bo, Assi nabbe pure Sau
So answer will be Akkad + bakkad + bambe + bo + 80 + 90 = 100.

Answer 2

There can be innumerable ways in which this problem can be done. 

One technique to solve the given problem is to use log of rational decimal values less than 1.

log(0.1) × 60 + log( 0.1 ) × 10  + 80 + 90 = 100  

Another Mathematical Solution: 

We are given :

4 blanks which sum up to ( 100 -90-80 = -70), taking 80 and 90 on RHS (Right Hand Side).

4 Blanks can't be negative. 

Now our new question is " _ +_ +_ +_ = -70" 

 The sum of the 4 numbers is negative and it is given that the given blanks can not be negative, so the 4 numbers can be complex numbers domains.
 
 A complex number is a number that can be expressed in the form a + bi, where a and b are real numbers. 

Complex numbers can not be compared to any number, so they are never negative and never positive. 

Answer (1) = (-70 + i) + ( 0 ) + ( 0 ) +( -i ) = -70

Answer (2) = ( 30 - i ) + ( -70 + i) + ( -30 + i ) + (-i) = -70 

Also Akkad + bakkad + bambe + bo + 80 + 90 = 100.