Because you need it in different units.
For instance, you know that you need to change the expression of fractions sometimes to do the math.
1/2-1/3 =?
-or-
You’d like to pay with quarters but all you have is nickels.
-or-
You want to feet to inches so you know how many squares you can make from cloth for use in a quilting project.
Those are simple examples and you can probably do them in your head. But they’re only one conversion. Maybe we can put them together into a more complex problem.
Three friends are doing a quilting project. Alice agrees to do half, Betty agrees to do a third, and eleanorigby agrees to be responsible for the rest.
eleanorigby can buy 2 foot x 3 inch strips of scrap cloth from a store in town for 75 cents each. She knows that the total quilt will be composed of 480 squares made from these cloths. Being the smart Doper that she is, eleanorigby sees her chance to unload the nickels that keep accumulating at her place. How many nickels does she need to pay for her cloth?
1/2-1/3= 3/6=2/6 = 1/6, eleanorigby’s responsibility.
480 squares x 1/6= 80 squares needed by eleanorigby
2 feet=24 inches, /3 inches per square=8 squares per strip needed by eleanorigby
8 strips x 75 cents/strip = $6.00
$6.00/.05=120 nickels needed
I could have set much of that up as one long continuous fraction to show the set of conversions but I handled each separately, which seems to be how you prefer it. That’s fine; you’ll get the same answer either way.
Another really complicated one that also involves multiple conversions:
Dopamine comes in 400mg per 250cc D5W (the D5W is unimportant). You receive a pt with a Dopamine drip infusing at 15cc/hr. Your pt weighs 112 kgs. You need to figure out how much Dopamine this pt is getting. The doctor wants the pt to get 5mcg/kg/min.*
That answer’s part smart-alecky and part serious. The doctor is probably following some sort of guidelines that are written in a medical journal that has treatment recommendations for mcg/kg/min but the medicine doesn’t come packaged in those units, has to be corrected for varying patient weights, and the doctor has to control the rate to get what he wants the patient to have, anyway.
And really, sometimes units are of very different magnitudes. The 400 mg is 400,000 mcg—in other words 80,000 times the 5 mcg target (they could have saved you a step and labeled it in mcg but it’s metric, so you only move the decimal around). Anyway dispensed over time and divided by the patient’s weight etc., it may be enough to do the trick. Or you may have to hang another bag.
To illustrate the point of using units of strange magnitude to measure things…
“You should sleep 8 hours a day” = “You should sleep 28,800 seconds per day”
“John is six feet tall” = “John is .00113636363 miles tall”
Some units are just more comprehensible for the circumstance. I don’t want to know how far it is for me to drive from here to work in inches, either. Each has its usefulness in different situations.
And can we jump systems:
She weighs 110 lbs vs. 50 kg
Body temp is 98.6F vs. 37C
I use lbs and feet and gallons but if it’s science, metric is the only way to go.
Converting numbers, even though you don’t change their value, allows you to get them all to play nicely together in the same sandbox.
You are so not getting the frosting, bucko. I speak as not only a girl, but the youngest girl AND the youngest child. I never got so much as a whiff of frosting…
