Money & Measurement

Two of the most practically important applications of numbers in daily life are money and measurement. You will encounter both constantly.

Money

What is Money?

Money is a system humans use to exchange value without directly trading goods. Instead of trading a chicken for a pair of shoes, you sell the chicken for money and use that money to buy the shoes.

Money works because everyone agrees it has value. This is a social convention maintained by governments and trust.

Currency

Different countries use different currencies (forms of money):

US Dollar (USD, $) — used in the United States; the world’s primary reserve currency
Euro (EUR, €) — used by 20+ European countries
British Pound (GBP, £) — used in the United Kingdom
Japanese Yen (JPY, ¥) — used in Japan
Chinese Yuan (CNY/RMB, ¥) — used in China
And hundreds more

Exchange rates determine how much of one currency equals another. They fluctuate based on economic conditions.

Coins and Bills

Physical money comes in two forms:

Coins — small metal discs, usually lower denominations
Bills/Notes — paper or polymer sheets, usually higher denominations

Digital Money

Most modern transactions are digital — no physical money changes hands. This includes:

Debit cards (connected directly to your bank account)
Credit cards (borrowed money you repay later)
Mobile payments (apps on smartphones)
Bank transfers

See Economics & Money for deeper context on financial systems.

Measurement

Humans have standardized systems for measuring physical quantities.

The Metric System (SI)

Used by most of the world:

Quantity	Unit	Abbreviation
Length	Meter	m
Mass/Weight	Kilogram	kg
Volume	Liter	L
Temperature	Celsius	°C
Time	Second	s

Common metric prefixes:

kilo- = 1,000 (1 kilometer = 1,000 meters)
centi- = 1/100 (1 centimeter = 0.01 meters)
milli- = 1/1,000 (1 millimeter = 0.001 meters)

The Imperial System

Used primarily in the United States (and partially in the UK):

Quantity	Units
Length	Inches, feet, yards, miles
Weight	Ounces, pounds
Volume	Fluid ounces, cups, pints, quarts, gallons
Temperature	Fahrenheit (°F)

🚩 This can cause confusion. Always confirm which system is being used, especially for medical or scientific contexts.

Temperature Conversion

°C to °F: Multiply by 9/5, then add 32. (e.g., 20°C = 68°F)
°F to °C: Subtract 32, then multiply by 5/9. (e.g., 68°F = 20°C)

Reference points:

0°C / 32°F = Water freezes
100°C / 212°F = Water boils
37°C / 98.6°F = Normal human body temperature
20°C / 68°F = Comfortable room temperature

Time

60 seconds = 1 minute
60 minutes = 1 hour
24 hours = 1 day
7 days = 1 week
~30 days = 1 month
12 months = 1 year
10 years = 1 decade
100 years = 1 century

Time is tracked on clocks (analog — with rotating hands, or digital — with numbers). The 12-hour format (1–12, AM/PM) is common in the US; the 24-hour format (0–23) is standard internationally and in science.