Well, 8 cakes use 200g, so you first divide by eight. You will then have how many grams of fat is used in 1 cake. (25g of fat in 1 cake) You want to know how many is in 3 cakes, so multiply by 3. (75g of fat in 3 cakes) There's your answer.
A proportion is good for solving this. You have 200g for 8 cakes, so how many (x) for 3? The proportion looks like this:
200/8. = x/3. Cross multiply 200·3 and 8·x. You get 600=8x. Divide by 8 to get x alone and you have x=75 grams for 3 cakes.
