Sugar.

Dry = 2kg

Wet = 1kg (water may dissolve the other kg)

Burn = 3kg (sugar oxidizes and gains additional weight coming from oxygen)

Step1:   Ignite both the ends of the first rope and only 1 end of the second rope. at the same time As soon as your first rope is burnt completely in 30 mins, you are left with 30 min of the other rope with one end unburnt.

Step2: Now ignite both the ends of the leftover 30 min length of rope 2, it will burn in 15 mins.