single input or seperate inputs for a solution containing units

We are trying to create a web application to teach units to students of 4-6'th grade, the question would be like

what is average velocity of the car that moved 60km in 3 hrs?

The answer would be 20 km/hr, one way is we can give single input so that student can enter 20 km/hr, another way is there will be 2 inputs, one input can only accept values and other can accept units. so,in first input the value would be 20 and the second input the value would be km/hr.

But another unusual thing is while we teach about currencies, the units input have to be placed left of the value, eg:$ 5.Due to these situations, I am unable to decide what input to use.

Most of the students use the web application in a phone, so Which way would be intuitive, or is there any other way to do this, I am new to UX, suggestions would be appreciated.

Thanks