Rate as an amount per unit

A shop sells identical notebooks. 4 notebooks cost 12 dollars. How much does 1 notebook cost? Let's draw it. One bar for the whole 12 dollars. The 12 dollars pays for 4 notebooks, so cut the bar into 4 equal units. One unit for each notebook. We want the cost of just 1 notebook. So share the 12 dollars over the 4 units. 12 divided by 4 is 3. So 1 notebook costs 3 dollars. That is a rate, and it is written 3 dollars per notebook. So to find the amount for each one, divide the total by the number of ones.

Your turn. Here's the question. A hose sprays three hundred and sixty millilitres of water in 3 minutes. How much water sprays out in 1 second? Try it first. Pause if you like. Let's finish it together. The time is given in minutes, but the question asks about seconds. 3 minutes is one hundred and eighty seconds. So 1 second gets three hundred and sixty divided by one hundred and eighty. That is 2 millilitres. 2 millilitres for each one second — 2 millilitres per second. One thing to notice. Shared over the 3 minutes, the hose sprays one hundred and twenty millilitres each minute, and that is a true rate too. But the question asked for each one second. Name the one, and the rate follows. So here is the rule to keep. A rate is an amount for each one. Cut the total into as many equal units as there are ones. One unit is the rate. And name the one. Each notebook, each minute, each second. The one you name decides the rate.