Edexcel IGCSE Maths 复习笔记 1.7.1 Basic Percentages

What is a percentage?

• “Per-cent” simply means “ ÷ 100” (or “out of 100”)
• You can think of a percentage as a standardised way of expressing a fraction – by always expressing is “out of 100”
• That means it is a useful way of comparing fractions e.g.

½ = 50% (50% means 50 ÷ 100)

⅖ = 40%

¾ = 75%

How do I work out basic percentages of amounts?

• You can use simple equivalences to calculate percentages of amounts without a calculator

• You can then build up more complicated percentages such as 17% = 10% + 5% + 2 x 1%

How do I find any percentage of an amount?

• You can find any percentage of an amount by dividing by 100 and multiplying by the given %
• Or you can use the decimal equivalent
  • Decimal equivalent = percentage ÷ 100
  • Percentage = decimal equivalent ✕ 100
• To find “a percentage of A”: multiply A by the decimal equivalent e.g.
  • 23% of 40 = 40 x 0.23 = 9.2
• To find “A as a percentage of B”: do A ÷ B to get decimal equivalent then x 100 e.g.
  • 26 as a percentage of 40 = 26 ÷ 40 x 100 = 65%