For part a, let's assume $10 as one part. One part of $10 gives me THB259. Now lets find out how many $10 parts $300 has. That's why we divide 300 by 10 and we get 30. Now we know $300 got 30 parts of $10. To find out how many THB we can get from $300, simply multiply 259 by 30, and we have the answer.

For part b, hire me and I'll teach you :)