A budget is a plan for money. Start with the amount available, add money received and subtract costs. Keep every number in the same unit. A receipt helps you check what was actually paid. These are pretend classroom amounts, not real purchases.
Worked example
Start with 200 cents and spend 65 cents. There are 200 - 65 = 135 cents left. Planning to buy something does not itself spend the money; update your record when a purchase happens.
Paper Craft Shop
You have 300 pretend cents. One paper pack costs 120 cents. One pencil costs 45 cents. You plan to buy one of each. Later a pretend helper gives you 50 cents. Work with these stated prices only; there are no taxes or extra charges in this exercise.