Two boats leave the same dock in cairo, illinois, at the same time. one heads north on the mississippi river whole the three heads south. the northbound boat travels 4 miles per hour. the southbound boat goes 8 miles per hour. how long will it take them to be 54 miles apart

4.5 hours

Step-by-step explanation:

Every hour they grow 12 miles part.

54/12=4.5

The time taken by them to be 54 miles apart is: 4.5 hours

Let it takes t hours for both the boats such that they are 54 miles apart.

The northbound boat travels 4 miles per hour.

Hence, the distance traveled by the northbound boat is: 4t miles.

( since the distance is the product of speed and time.

i.e.

Distance=Speed×Time)

The speed of the southbound boat is: 8 miles per hour.

This means that the distance traveled by this boat in t hours is: 8t miles

Hence, the total distance between the two boat in t hours is:

4t+8t=54

i.e.

12t=54

i.e.

t=54/12

i.e.

t=4.5 hours

                    Hence, the answer is: 4.5 hours

let x and 400 - x be the two numbers.  

==> 0.80x + 0.85(400 - x) = 400 - 68 from the statement of the problem.  

< ==> 0.80x + 0.85(400 - x) = 400 - 68  

==> 340 - 0.05x = 332  

< ==> -0.05x = -8 ==> x = 160, the first number ==> 400 - x = 240, the second number.