This is a simple GCSE question which might feature at the beginning of the paper. It tests the student's attention to detail by asking to find the DIFFERENCE and not just the price that Sue paid. Also, given that this could appear in a non-calculator paper, it also tests pupil's ability to work with decimals.
Firstly, we start by working out how much has Sue paid for train tickets, given that she went to work 18 times and bought a "day return" ticket. Hence, 18*£6.45 gives us £116.1. Then, we need to find the difference between a monthy saver ticket and 18 days worth of day return tickets. Therefore, our final answer should be £116.1-£98.5=£17.6.