Question 10... The answer to this would be $\displaystyle \frac{b}{x}\times y$. Dividing the cost by the number of apples $\displaystyle x$ works out the cost per single apple. Then multiplying this value by $\displaystyle y$ gives the cost of $\displaystyle y$ apples.

For question 11, since 25% is a quarter of 100%, 12 students must be a quarter of the class. So to calculate the size of the class, simply do $\displaystyle 12\times 4=48$.

Question 12... Okay, I'm not good at explaining things like this, and I'm not even sure my answer's right. All I did was $\displaystyle 0.25 \times 0.6=0.15$ (this calculates the proportion of the graduating class that took 6 or 7 courses i.e. 15%), then I added this to the ones who did 8 or more maths courses, so 75% + 15% = 90%, meaning the remaining 10% must do fewer than 6 maths courses. So the answer's 10% I think.