Hi everyone, I have a homework question which I don't know how to answer. Here is the question:

Harry is walking in a marathon. He was intending to complete the marathons total distance of 42km in seven hours while keeping a steady pace of 6km per hour. He was going good for his first four hours but developed a cramp in his legs and stopped to rest for 18 minutes. By what percentage will he need to increase his walking speed for the rest of the marathon if he is still going to complete it in seven hours? Include steps to your answer.

Now, I'm not sure how to calculate this. I subtracted 18 from the last 3 hours of the marathon and got 2 hours and 42 minutes. I then converted this to a decimal (2.7) and then to a percentage (38.6 2s.f) but I'm not sure I'm on the right track. Another problem is that I'm supposed to incorporate algebra into my answer and I have no idea how to do so. Anybody willing to help me out and confirm whether I'm correct or not on my earlier calculation? Thanks all.