38.5 mol of P4O10 contains how many moles of P?

The subscripts tell the ratio of one element to another in a compound. In this case, there are 4 P atoms per 10 O atoms. This could also be interpreted as 4 mol P per 10 mol O. Therefore:

Mol P = (38.5 mol P4O10) / (4 mol P / 1 mol P4O10) = 154 mol P