Without doing exact calculations, you can round the price of a bag of candy to $2 (rounding up is your safest bet, since it takes sales tax into consideration and gives you an estimate on the high side) and quickly determine that 3 bags of candy would be about $6 and 4 bags would be about $8.
Order of magnitude is usually written as 10 to the nth power. The n represents the order of magnitude. If you raise a number by one order of magnitude, you are basically multiplying that number by 10. If you decrease a number by one order of magnitude, you are basically multiplying that number by 0.1
