Tnk uses another formula for calculating the current through two parallel resistors.I agree with all the points except 4 and 5.
I through the right resistor:
current divider : I= Req/R Isrc
Req= 10 left resistor// (10right resistor +30) = 8 ohm
R= (10 right resistor +30) = 40 ohm
Isrc= 5 A