Money is decimals with two places and a dollar sign, and the reliable way to keep it exact is to think in cents: $1.70 is 170 cents, and 170 is a whole number that cannot go wrong in a division. Most P6 money questions are the unitary method — divide to find what ONE costs, then multiply to find what you want — and that is also how you compare two shops selling different-sized packs, because $5.00 and $7.20 tell you nothing until you know how many each pack holds. Tariff questions (car parks, taxi fares, courier charges) look different but are the same idea in two halves: work out the duration or the quantity first, then charge the first part and the extra parts separately.
Worked example
Problem 8 identical exercise books cost $13.60. Find the cost of 5 such books.