Proportionality calculations lead to the use of the so-called 'rule of 3', also nammed the 'cross product law'.

Let's study it with an example.

If it lasts 15 minutes to bake 500 g of rastbeef, how much time shalll I bake my 950 g rastbeef?

Let's call * x* the time in minutes that we are looking for.

Then the time per gram is either 15 min over 500 g, or x min over 950 g:

* x* 15

------ = ------ (1)

950 500

If we multiply...