I couldn't figure this out!

You should really define your variables.

Let x = the number of ml of 20% acid that need to be added.
Let y = the number of ml of 100% acid that need to be added

x+y = 50ml
What is the 2nd equation?

You ask what is wrong with your work--you are assuming that 50ml is the amount of the pure acid which is not true.

Also 20x + 50 = 45 makes no sense at all. The x and 45 is a percentage while the 50 is a number of ml. For example, can 40ml + 30ml = 70%? NO! 40ml + 30 ml = 70 ml.
 
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