(I) If the ratios are directly proportional, then

5 : 6 = x : 15.6

(II) If the ratios are inversely proportional, then

5 : 6 = inverse of x : 15.6 = 15.6 : x

Points to Remember:

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• If a:b is directly proportional to c:d, then a:b = c:d or ad = bc. (by cross multiplication)

• If a:b is inversely proportional to c:d, then a:b = inverse of c:d = d:c or ac = bd. (by cross multiplication)