To know which bag is cheaper, you have to compare both package price as cost per kilograms of sugar. Then we can find how much we can save by finding the difference between the two values
The 500g sugar package cost is N195. Every kg is equal to 1000g, so the price will be: 195N/ (500g*1kg/1000g)= 390 N/kg
The 25kg sugar package cost is N8750. so the price will be: 8750N/25kg= 350 N/kg
The amount you save per kg if you buy 25kg package instead of 500g will be: 390 N/kg - 350N/kg = 40 N/kg
