[IceRiceIce]

How do I do derived units? For example: (100g)/(15mL),    (340 km)/(175 s),    (25kg)(6.0 m)/ (15 s)(20 s), 
(41.7 kg)/(1.3dm * 3.4dm * 2.1dm), (2.45 mol)/(1.25 liters)

[chenbeier]

You have to calculate them,to get SI units.

[IceRiceIce]

You have to calculate them,to get SI units.

Yes but how do I calculate them? Is it algebraically?

[Borek]

You already asked very similar question. There is no such thing as "doing derived units" - calculations have some purpose and the purpose isn't clear here.

If 15 mL of something have weight of 100 g you can calculate its density - which is typically expressed in g/mL. You will get the density expressed in g/mL if you simply divide 100/15. But that will produce an answer to the meaningful question "calculate the density" and not to the meaningless question "do derived units".

[MNIO]

I.R.I... did you try googling "definition derived units" so that you understand what Borek is telling you?
https://en.wikipedia.org/wiki/SI_derived_unit.
if you take 1 unit and divide by another (or multiply by another.. or mult/div of any number of units), you get a "derived unit"
there are some common derived units.  See the link. 

[Meter]

You have to calculate them,to get SI units.

Yes but how do I calculate them? Is it algebraically?

It can be done if you understand how to convert each part of the unit. For example, you that there is 0.001 kg for every 1 g and you know that are 0.001 L for every 1 mL. In this example, you can simply multiply the grams with 1000 and the mL with 1000 to get the equivalent numerical amount in kg/L. For example, 100 g/mL is the same as 100 kg/L. This one is easy because both are metric units.

We may take a slightly harder unit like km/s. We want to convert this to m/s (SI-unit). So we only have to manipulate the numerator, since the denominator is already expressed in our desired unit! We know that for every km, we have 1000 m. So we must multiply the numerator by 1000 to go from km to m. We see that 1 km/s then corresponds to 1000 m/s.

Let's consider an even harder example, we may want to convert km/h to m/s. Now we have to manipulate both the numerator and the denominator. But as long as we just convert each unit independently, it is not that hard. As demonstrated before, we can go from km to m by multiplying by 1000. How do we go from hours to seconds? Similarly, we understand that for every hour, we have 60 minutes, and for every minute we have 60 seconds. This means that 1 hr = 60*60 s. So let's say we have a velocity of 1 km/h. Then we must multiply this number by 1000/(60*60) and we get that 1 km/h = 0.278 m/s (approximately).

To summarize:

1. Look at each part of the unit independently.
2. Determine what you have to multiply each unit with to convert it to the desired unit.
3. Multiply the actual quantity with this number.

I hope it's clear!