Tie both ropes together, light them on fire, and when they are 3/4 burned....
45 minutes.
(I did not look at the answer first. FWIW)
Problem says burn rate is not constant, so you cannot time 45 minutes by any ratio.
Fold them in half... fold them a second time. Tie them together where a matching end fold was. Light the longer free ends of both. When the burns hits the knot... 45 minutes.