Rates — pay, exchange, flow, fuel, pressure, density, average speed

A rate is "top quantity per bottom quantity", so density is mass $\div$ volume — not volume $\div$ mass. Putting the two numbers into the calculator in the wrong order gives the reciprocal of the right answer.

For average speed in km/h the time must be in hours. A time of $3$ hours $45$ minutes is $3.75$ hours, not $3.45$ hours — the $45$ minutes is $\tfrac{45}{60}=0.75$ of an hour. Treating the clock reading as a decimal is the most common slip.

For a multi-leg journey the average speed is total distance $\div$ total time, not the average of the leg speeds. Averaging the speeds only works if every leg takes the same time; otherwise it gives the wrong answer.

If the question asks for litres per minute, an answer in litres per hour is wrong — convert first. Check whether the rate is wanted per second, per minute or per hour before you divide, and match the answer to it.

Dividing by a decimal such as $0.2$ is not the same as dividing by $2$: $1500 \div 0.2 = 7500$, not $750$. Watch small decimal divisors — dividing by a number below $1$ makes the answer larger.