A book has (10) questions in the first chapter and (4) more questions in each next chapter. How many questions are in the (9)th chapter?

Correct answer: 42

The number of questions increases by 4 in each successive chapter, so it forms an arithmetic progression. Here, \(a=10\), \(d=4\), and \(n=9\). Thus, \(a_9=a+(n-1)d=10+(9-1)\times4=42\). Therefore, the correct answer is 42. Getting 40 would count only 7 increases, whereas there are 8 gaps from the first to the ninth chapter. Exam tip: for the \(n\)th term, always use \(n-1\) common differences.