Question 311231
First boat--> rate    *    time    =  distance
               10     *      t     =    X 
                           

Second boat-->rate    *    time    =  distance
               8      *     t      =    54-X 
                        

Solve first boat for t:
               10t = x
             10t/10= X/10
                 t = X/10

Substitute into equation for second boat:
             8t = 54-X
            8(X/10)=54-X
             8X/10 = 54-X
             4x/5 = 54-X
             5(4x/5) = 5(54-X)
               4x    = 270-5X
               9x    = 270
                x    = 30

Substitute new X value into equation for boat #1:
             10t = X
             10t = 30
               t = 3

In three hours the two boats will be 54 miles apart.