Rule of three calculator
Type of proportion
direct – more of one means more of the other
inverse – more of one means less of the other
Known value a
e.g. 3 (lb)
corresponds to value b
e.g. 4.50 ($)
New value c
x is the value that goes with c, e.g. 5 (lb)
Result
JavaScript is required for the calculation.
Answer: x (goes with c)
7.5
Value for 1 (unit value)
1.5
Working
Given: 3 → 4.5
1. Find the value for 1 (divide): 1 → 4.5 ÷ 3 = 1.5
2. Scale up to 5 (multiply): 5 → 1.5 × 5 = 7.5
Formula: x = b ÷ a × c = 7.5
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