Thank you for your answer. My final answer is 42.

I’m confused why if after she gave 5 of each ribbon away, why would we need to do 1u + 5 + 20, then add back another 10.

- if she has 50 as your answer and the model answer I’m looking at is, after she gives 10, she has 40, then adds another 40 ribbons for 80. However 80 can’t be divided by 3 parts (in the end 1/2 as many purple as blue)

Is this a problem with me mis-interpreting the question?

Wait...Let me digest your doubt...

At 1st is 50.
IF she gave away 10 mean not really gave away, just assuming only.

Later she bought another 40
Total in the end = 90
Divide by 3 will 30 each. Blue = 30 Green =60

To clarify, I accept 50 as the correct answer from multiple sources including yours btw. I just cant wrap my head around it as I model it out or work backwards from the answer. As such I am struggling to pass the concept on to the child.

Thank you for your time and the double response.

I uploaded another answer using model method. Hope to clear your doubts. Ratio method as teach in school using difference remain unchanged (constant difference as described in your post).