Question 258414
speed * time =distance
also written as r*t=d
We know the time and the distance
Surely, if I told you that I went 60 miles in one hour you would say  that I went 60 mph.
x*1=60
x=60
or if I told you that I went 120 miles in 2 hours you would know I went 60 mph.

x*2=120
x=60 again

But something is wrong with your problem.
It says"Find the speed of the boat and the speed of the plane. "
There is no plane mentioned.
r*2=46
r=? You can solve this!
and the other 
r*3=51
r=51/3=?

so it should current not plane the current is 3 
20+3=23
20-3=17