Your questions deal with the concept of % changes or increase/decrease % (or inflation/discount).
In all such case, new cost (or price) = old cost (or price) + old cost (or price) x % change
remember that positive change or increase is given by a positive sign and a discount or decrease by a negative sign.
I will solve just a couple of your questions to give you the idea and hopefully thy will help you get through the rest on your own...
1) Buy & sell scenario: Net cost = buying price -selling price = 12000 - 10000 = 2000 euros.
Rent option = 195/month x 12 months/year = 2340 euros
with 15% discount: cost of rental option = 2340 + 2340 x (-15/100) = 1989 euros.
So its cheaper to rent it.
2) Old price = 65 euros
sale discount = 15%
after discount cost (or final cost) = 65 + 65x (-0.15) = 55.25 euros
3) Cost of electrical supply = 250 + 250 x (-0.7) = 175 euros
cost of new house = 240 x 2/3 = 160 euros
Cost of fridge = deposit + monthly cost summation = 50+ 20x6 = 170 euros
Thus the new house is the cheapest
Rest of the questions can also be similarly completed.