Hint: In this particular question use the concept that in 1 kg there are 1000 grams so the sum of all the elements in the mixture is equal to 1 kg, so use these concepts to reach the solution of the question.
Complete step by step answer:
A namkeen mixture contains 250.785 gm roasted chana, 225.802 gm roasted moong and 300.005 gm roasted peanuts.
Now we have to find out the grams of roasted corn flakes should be added so that the weight of the mixture is 1 Kg.
As we all know that in 1 kg there are 1000 grams.
Let X grams of roasted corn flakes are added in the mixture.
So the sum of all the elements in the mixture is equal to 1 kg i.e. 1000 grams.
Therefore, roasted chana + roasted moong + roasted peanuts + roasted corn flakes = 1000 grams.
Now substitute the values we have,
$ \Rightarrow 250.785 + 225.802 + 300.005 + X = 1000$
Now simplify it we have,
$ \Rightarrow X = 1000 - 776.592 = 223.408$ Grams.
So we have to add 223.408 gm of roasted corn flakes.
Note: Whenever we face such types of questions the key concept we have to remember is that assume any variable be the amount of roasted corn flakes in the mixture so add all these questions and equate to 1000 gm as above and simplify we will get the required amount of cornflakes in grams which should be added in the mixture.