You dont have to think in exact numbers when you do this... I think that people do too much maths and not enough real thinking when they do electronics.
This circuit is so basic, why bother calculating any number at all.
So you can see we have 1 led on the left with a resistor, and 3 leds on the right, 2 in parallel 2 in series, with no resistance, so the right is definitely alot brighter than the left, and if you used a small enough battery then you wouldnt be able to even see anything on the led with a resistor, regardless of the resistance.
Thats worth more any amount of pages of confusing mathematics, and gets it over and done quicker, and you dont have to worry about pesky details that dont matter.
There is more to conclude here and know than just that, like the led voltage drop, you need to be able to handle that as well... but that probably can be "read" as some approximate laws and the exact numbers of things isnt so important, the general concepts are way more important.