Alright, you could approach it like this. You need a 240 ml of 20% solution. How much “stuff” is there in that quantity at that concentration? 0% is no stuff, 100% is pure stuff, etc. So, 20% of 240 ml is 48 ml of pure stuff. Now, if you had the pure stuff (100% solution) at hand, and you mixed 48 ml of that with 192 ml of diluent (0% solution), you would have what you need - a total of 240 ml of which 48 ml (i.e. 20%) is stuff.
Now, you don’t have pure stuff at hand, you only have 50% stuff solution. You still need to get 48 ml of pure stuff in the final mix. How much 50% stuff solution does it take to get 48 ml of pure stuff. Well, since it’s half diluted, you need twice as much, so 96 ml of 50% solution will contain the 48 ml of stuff, plus 48 ml of diluent. Now, you still need to fill up to 240 ml. So, 240-96=144. You need to add 144 ml of diluent (0% stuff) back to get the volume you need.