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Ratios, Rates and Proportions — Digital SAT SAT Math revision notes

A 3-page visual revision sheet for Ratios, Rates and Proportions, written for Digital SAT SAT Math. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.

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Ratios, Rates and Proportions in one look

one relationship held constant while the numbers are scaled up and down

What makes a relationship proportional

  • two quantities are proportional when their ratio never changes. Triple one and the other triples too
  • every proportional relationship can be written y=kx, where k is the constant of proportionality, the amount of y that goes with one unit of x
  • a RATE is a ratio with different units on top and bottom: pages per minute, rupees per litre, people per square kilometre
  • a UNIT rate is that ratio rewritten with 1 in the denominator, which is what makes it easy to scale to any other amount

Three ways to write the same thing

  • the equation form, k constant y=kx
  • the constant read off any one pair k=y/x
  • the proportion form, then cross multiply a/b=c/d⇒ ad=bc

Worked: a constant rate

Q. a machine fills 84 bottles in 6 minutes. At the same rate, how many bottles does it fill in 15 minutes?

  1. unit rate first: 84/6=14 bottles per minute
  2. scale to the time asked for: 14Ɨ15=210
  3. or scale the whole block instead: 15 minutes is 2.5 times 6 minutes, so 2.5Ɨ84=210

Answer: 210 bottles

The three shapes this topic comes in

  • Rate: an amount per one unit of something else. Find the unit rate, then multiply
  • Proportion: two ratios set equal with one unknown. Cross multiply and divide
  • Scale: a drawing, map or similar figure. Every matching length shares one scale factor
  • Sharing: a total split in a given ratio. Add the parts, divide the total by that

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