In statistics(eco) class 11, I have a doubt in the chapter index numbers. In Q18 of the ncert exercises, why is the sum of the weights considered to be 100?

Sum of the weights is considered to be 100 because in the question you need to find percentage of the total weight given to food. In order to find percentage(i.e 100) of the total weight given to food and other items it is necessary to assume sum of weights as 100.