Question 859490
A ship travel from port A to port B with distance 20 km in 30 minutes.
the ship travel back from port B to port A in 20 minutes.
 What is the average speed of the round trip in km/hr.
:
Let a = average speed for the trip in km/h
:
Assuming the one-way distance is 20 km
Change 30 min to 1/2 hr, and 20 min to 1/3 hr
:
Find the speed  A-B and B-A
{{{20/(1/2)}}} = 40 km/hr
:
{{{20/(1/3)}}} = 60 km/hr
:
Write a time equation; time = dist/speed
{{{20/40}}} + {{{20/60}}} = {{{40/a}}}
Multiply by 120a to clear the denominator, results
3a(20) + 2a(20) = 120(40)
60a + 40a = 4800
100a = 4800
a = 4800/100
a = 48 km/hr av speed for the round trip