Two ropes , X & Y.
Cut rope Y in 3 pieces, Y1,Y2 & Y3
Light rope X (one endpoint) , and all the endpoints of Y1 , Y2 & Y3.
At the time a piece Yn burns out, cut one of the remaining Y pieces, and light the 2 new endpoints.
When all the pieces Yn burns out, start the clock.
When X burns out, stop the clock.
50 minutes elapsed !
-----------
However, this solution is not implementable.