I'm humbly requesting some aid in this exercise.

The question is at what value of U1 does the red/yellow LED activate.

I did a simple division of the voltage and got 8.9V as the 'pivot', and over/above that as activating red/yellow. My mate went to a study group and got the answer 4.8V as the 'pivot', which is more likely to be correct. But I really don't get how.

Anyone here that know much about electronics that can lend a hand?