… [I've been given heights] 20 20 20 20 [to add]; so I imagine [each] as something with size 20 in real life and get stuck because in real life we always have errors and not measuring pieces exactly …]

Precision depends on context.

If your exercise models heights and you're

**given** four heights as 20 units each, then consider each height as exactly 20 units high (regardless of the real world).

Don't think about changing given numbers or concerning yourself with reasonableness of models in any exercise --

*unless* you're specifically asked to do those sorts of things.

**Many exercises model things in the real world by using ideal conditions** and rounded measurements and exceptions to reality. These exercises are only for practice; focus on things like rules, basic formulas, properties, and methods. Be more concerned with accuracy (does your answer make sense), not precision (is it exact to the nth degree, using advanced rules for measurement taking, standard calibrations and the concept of significant figures).

Once you get into subjects using applied mathematics (eg: physics, biology, economics, engineering), then you'll be learning about precision and how it affects mathematics in the real world. Until then, remember that most real-world exercises you get now will be idealized, so as to focus more on meaning and less on how precise the answer needs to be.

Find the stacked height of four shipping containers, where each container measures 20 units high.

20 + 20 + 20 + 20 = 80

That's accurate! Who cares whether or not it's a precise model of heights in some actual, real-world situation. The exact answer to the exercise is: 80 units. Whether the measurements deviate in any significant way from reality is a question for somebody working in the real world to worry about. :cool: