Same Unit of measurement in an equation

You can add or subtract two quantities only when they are measured in the same unit.
Adding 5 meters and 3 kilograms makes no sense, because length and weight are completely different things.
So the first step is always to check that the units match.

When two quantities use different units of the same kind, convert them to one unit first.
To add 2 hours and 30 minutes, change 30 minutes to $\frac{1}{2}$ hour, giving $2 + \frac{1}{2} = 2.5$ hours.
Only after converting can you safely add.

In any equation, both sides describe the same quantity, so they carry the same unit.
Say a taxi charges a $3 flat fee plus $2 per mile, so the total cost is $C = 3 + 2m$.
Since $C$ is a number of dollars, both $3$ and $2m$ must also be dollars.

That means a number in front of a variable often carries a hidden unit.
In $2m$, the $m$ is a number of miles, so the $2$ must be dollars per mile.
Then dollars per mile times miles gives dollars, which matches the total — the units line up.

Can you add 200 grams and 3 kilograms directly?
No — the units are different, so convert first.
Since 3 kilograms is 3000 grams, the total is $200 + 3000 = 3200$ grams.

A phone plan costs $20 plus $0.10 per text, so the total cost is $C = 20 + 0.1t$. What unit does the $0.1$ carry?
The total $C$ is in dollars and $t$ is a number of texts, so $0.1t$ must be dollars.
So the $0.1$ is dollars per text.

In the equation $d = 60t$, distance $d$ is in miles and time $t$ is in hours. What unit must the $60$ have?
Both sides carry the same unit, miles, and $t$ is in hours, so $60t$ must be miles.
So the $60$ is miles per hour — a speed.