Question 188954
Let x=speed of the current
    30-x = speed of the boat upstream
    30+x = speed of the boat downstream
    t = time it takes to go downstream

If it takes 1 and 1/2 times longer to go upstream over the same distance, and distance=speed * time

{{{(30-x)*1.5t=(30+x)*t}}}

{{{45t-1.5xt=30t+xt}}}

{{{15t=2.5xt}}}

Dividing both sides by 2.5t to solve for x, we get:

{{{6=x}}}

The speed of the current is 6mph