The rate formula
The definition on the last page can be written as a formula. Since it can be written from either side of the equation, the reactants or the products, there are two of them.
From the reactants
rate of reaction = amount of reactant used up ÷ time taken
From the products
rate of reaction = amount of product formed ÷ time taken
What counts as an "amount"?
Whatever is easiest to measure — usually a mass or a volume. If the reactant is a solid its mass can be weighed; if the product is a gas its volume can be collected and measured.
An example
If 3.2 g of limestone is used up in 4 minutes,
3.2 g ÷ 4 min = 0.8 g min−1
The unit depends on what we chose to measure — grams per minute, cubic centimetres per second, and so on. What matters is that it always takes the form "so much of something, per unit of time".