Question 451550
Formula for distance is r*t=d where


- r is a rate (speed) of movement of the object
- t is a time measured during the movement of the object
- d is a distance traveled during the movement of the object

We have to take in the rate of the river current - c.


So for the downstream we have:


(r+c)*t=d


Going upstream Noah travels against the current of the river, which rate we can mark as c) so the distance becomes 


(r-c)*t=d, river slows down Noah


If we plug all the values we know we get


(r+c)*1.5=30
(r-c)*1.5=12


Dividing both equations by 1.5 we get


r+c=20
r-c=8


If we add these two equations we get


2r=28
r =14km/h, rate for Noah


Go back to find rate of the current:

r+c=20
c  =20-r
c  =20-14
c  =6km/h


Rate for the river current is 6km/h.