ZS Associates Technology Analyst interview questions

You have two ropes coated in an oil to help them burn. Each rope will take exactly 1 hour to burn all the way through. However, the ropes do not burn at constant rates—there are spots where they burn a little faster and spots where they burn a little slower, but it always takes 1 hour to finish the job. With a lighter to ignite the ropes, how can you measure exactly 45 minutes?