A submarine travels 16km/h on the surface of the water and 10km/h under water. The sub covers 138km in 12 hours.

I) Write 2 equations to represent; (Define Variables) total number of
hours:

Total distance travelled.

II) Use equations to calculate the spent on the surface and the time spent under water.

I) To represent the total number of hours, let's define two variables:

Let 'x' represent the time spent on the surface (in hours).
Let 'y' represent the time spent underwater (in hours).

To represent the total distance traveled, we can use two equations:

Equation 1: Distance traveled on the surface
The speed on the surface is 16 km/h, so the distance traveled on the surface can be calculated by multiplying the speed by the time spent on the surface:
Distance on surface = 16 km/h * x hours

Equation 2: Distance traveled underwater
The speed underwater is 10 km/h, so the distance traveled underwater can be calculated by multiplying the speed by the time spent underwater:
Distance underwater = 10 km/h * y hours

II) Now, let's use these equations to calculate the time spent on the surface and the time spent underwater.

We are given that the total distance covered is 138 km and the total time spent is 12 hours. We can set up the following equation to represent this:

Distance on surface + Distance underwater = Total distance
(16 km/h * x hours) + (10 km/h * y hours) = 138 km

We also know that the total time spent is 12 hours:

x hours + y hours = 12 hours

Now we have a system of two equations with two variables. We can solve this system to find the values of 'x' and 'y', which represent the time spent on the surface and the time spent underwater, respectively.