Apprendre
Trouver des valeurs inconnues lorsque deux quantités sont dans un ratio donné.
Ratio problems express a relationship between two quantities. If A:B = 2:3 and you know A=10, you can find B by first finding the multiplier (10÷2=5) and then applying it to the other part (3×5=15). The ratio tells you the relative sizes, and the multiplier scales it to actual values.
If A:B = m:n and A = x, then B = x × (n/m)
- If A:B = 2:3 and A = 10, then B = 10 × (3/2) = 15
- If X:Y = 3:4 and Y = 20, then X = 20 × (3/4) = 15
- If P:Q = 5:2 and P = 35, then Q = 35 × (2/5) = 14
Comment le reconnaître
- Two quantities described as 'in the ratio' of X:Y
- One value is given, the other is unknown
- Find the multiplier by dividing the known value by its ratio part
Erreurs fréquentes
- Dividing by the wrong ratio part
- Confusing which value corresponds to which part of the ratio
- Adding/subtracting instead of multiplying
Résolution étape par étape
Le ratio garçons/filles dans une classe est 3:5. S'il y a 12 garçons, combien de filles y a-t-il ?
- Identifiez les parts du ratio : garçons = 3, filles = 5
- Trouvez le multiplicateur : 12 garçons / 3 parts = 4
- Appliquez le multiplicateur pour les filles : 5 parts x 4
- Calculez : 5 x 4 = 20 filles
Réponse: 20 girls