To answer this question, firstly work out how long it will take to cover the 100nm at the current speed.

From the data given, the current TAS is 242kt, which makes the current GS 272kt. Thus, to cover 100nm at 272kt will take 22mins.

So, to arrive there 5 minutes later, you need to be taking 27mins to cover 100nm.

Working backwards from this makes the revised GS 222kt, so the revised TAS would need to be 222-30=192kt.

Going back to your nav computer & with the same data in the airspeed window, locate 192kt on the outer scale and read off the inner scale the revised airspeed required - i.e. 159kt.