ACCA FM Specimen Exam – Forecasting Foreign Currency

Hello,

I am trying to understand the rationale for the answer in the ACCA FM Specimen Exam.
Per the free lecture notes/lecture, we should be calculating the future exchange rate to be
S1 = S0(1+Hc)/(1+Hb) (Using inflation figures)
OR
S1 = S0(1+Ic)/(1+Ib) (Using interest rate figures)

Q1 Specimen exam has both inflation & interest figures, both of which provide different answers. Which figures should be used?

TIA.