"Bob spent twice as much as James, who spent four times as much as Nava"
Bob: b
James: j
Nava: n
Bob spent twice as much as James: b=2j
James spent four times as much as Nava: j=4n
Bob and James together spent $180: b+j=180
but we have: b=2j
-------------------
2j+j=180
3j=180 >> j=60$
b=2j=120$
Recall this: j=4n >>>> 60=4n >>>>>n=15$
SO far we calculated how much everyone spent
I hope that helps!