Rule of 3 Calculator

Rule of 3 Calculator

Direct (If A increases, B increases) Indirect/Inverse (If A increases, B decreases)
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Understanding the Rule of Three

The Rule of Three is a mathematical method used to find an unknown fourth value (X) when three other values are known, provided those values maintain a constant ratio or proportion. It is one of the most practical mathematical tools used in everyday life, from cooking and shopping to engineering and chemistry.

1. Direct Rule of Three

This is used when the relationship between variables is directly proportional. This means if one value increases, the other increases at the same rate.

Example: If 3 liters of fuel cost 6 euros, how much do 10 liters cost?
A = 3, B = 6, C = 10.
Formula: (B × C) / A → (6 × 10) / 3 = 20 euros.

2. Indirect (Inverse) Rule of Three

This is used when the relationship is inversely proportional. If one value increases, the other decreases.

Example: If 2 workers can paint a house in 10 hours, how long will it take 4 workers?
A = 2, B = 10, C = 4.
Formula: (A × B) / C → (2 × 10) / 4 = 5 hours.

Pro Tip:

Always ensure your units are consistent. If Value A is in "hours", Value C must also be in "hours" for the calculation to be accurate.

function calculateRuleOfThree() { var valA = parseFloat(document.getElementById('valA').value); var valB = parseFloat(document.getElementById('valB').value); var valC = parseFloat(document.getElementById('valC').value); var type = document.getElementById('propType').value; var displayX = document.getElementById('displayX'); var explanation = document.getElementById('calc-explanation'); var formulaText = document.getElementById('formulaText'); if (isNaN(valA) || isNaN(valB) || isNaN(valC)) { alert('Please enter valid numbers in all three fields.'); return; } if (valA === 0 && type === 'direct') { alert('Value A cannot be zero in a direct proportion.'); return; } if (valC === 0 && type === 'indirect') { alert('Value C cannot be zero in an indirect proportion.'); return; } var result; var mathString; if (type === 'direct') { // Formula: X = (B * C) / A result = (valB * valC) / valA; mathString = "Calculation (Direct): Since " + valA + " leads to " + valB + ", then " + valC + " leads to X.Formula: (Value B × Value C) / Value A = (" + valB + " × " + valC + ") / " + valA + " = " + result.toLocaleString(undefined, {maximumFractionDigits: 4}) + ""; } else { // Formula: X = (A * B) / C result = (valA * valB) / valC; mathString = "Calculation (Indirect): Since " + valA + " corresponds to " + valB + ", then " + valC + " corresponds to X.Formula: (Value A × Value B) / Value C = (" + valA + " × " + valB + ") / " + valC + " = " + result.toLocaleString(undefined, {maximumFractionDigits: 4}) + ""; } displayX.innerHTML = result.toLocaleString(undefined, {maximumFractionDigits: 4}); formulaText.innerHTML = mathString; explanation.style.display = 'block'; }

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