given 2 non-uniform ropes (each burns down in 1hr, but not uniformly), how to measure 45 mins

Ignite one rope on both the ends. Ignite the second rope at only one end. When the first rope burns out, it is 30 mins. Then ignite the unburned end of the second rope. When the remaining rope burns out, it is another 15 mins. So it is 45 mins.