Answer:
Because they both burn inconsistently, you cannot simply light one end of a rope and wait until it’s three quarters of the way burnt. But, this is what you could do: Light the first rope at both ends, and light the other rope at one end, both at exactly the same time. The first rope will take 30 minutes to burn because even if one side burns faster than the other, it will still take 30 minutes. The exact moment the first rope goes out, light the other end of the second rope. Because the time elapsed of the second rope burning was 30 minutes, the remaining rope will also take 30 minutes. Lighting it from both ends will cut that in half to 15 minutes, giving you 45 minutes all together.