F.1 math 聽日考試啦，唔該幫幫忙

Let n be the exchange rate
￡30 x n = HK$504
n = 16.8

Let X be the HK dollars needed for exchange

X = ￡100 x n
X = 100 x 16.8
X = HK$1680

￡30 for HK$504

504/30 x100 = 1680
￡100 for HK$1680