Hi Michel,

Let B = base amount = $500
T = tax amount = $74.88
F = Federal tax % = 5
P = Provincial tax % = 9.5


if we know T, F and P then B can be derived as follows:

B = (T*100) / ( F + P + (F*P)/100)

Having derived B you can the calculate the Federal tax amount as B * (F/100)
and the Provincial Tax amount as B * (1+(F/100)) * (P/100)

HTH,

Andy