CFAI Reading 18 Q18b & Q19b

I am somwhat confused by the answers for the two questions. Q18b I agree with what is done in the CFAI book. Q19b disagree. The base currency is SFR. So we should find the midpoints and invert the midpoint of the given quotes. The CFAI answer assumes $ as the base currency in the solution. It may be that my brains are fried right now and there is some language that I am misinterpreting. I appreciate any comments. Thanks.