Susan Brown
Our question is “In order to calculate…”, so yes, the total resistance in the circuit is the first step. And, it’s a very easy one, since we’re just talking about adding up numbers.
Part of the point of this question is to get people comfortable with simple calculations involving voltage, current, and resistance.
As to using an ammeter instead… our context is that of appliance repair. We are doing measurements on an appliance that has a malfunction. We do electrical measurements in order to compare them to specifications, so first we need those specs.
Specifications come from the technical documents, and sometimes the specs you need are conveniently listed, sometimes they aren’t. When they aren’t, it’s handy to be able to do a quick calculation to get the spec you want from the info they give you.