Answer:
13/20
Step-by-step explanation:
To add fractions without the common denominator, you have to make them equal so
1/4 x 5 = 5/20
2/5 x 4 = 8/20
5/20 + 8/20 = 13/20
Check the picture below.
now, keep in mind that ship B is going at 20kph, thus from noon to 4pm, is 4 hours, so it has travelled by then 20 * 4 or 80 kilometers, thus b = 80.
whilst the ship B is moving north, the distance "a" is not really changing, and thus is a constant, that matters because the derivative of a constant is 0.
If you use the distance formula you get 10.