| Difficulty | Secondary School | High school/other |
| ★ | Basic | Easy |
| ★★ | Demanding | Basic |
| ★★★ | Demanding | |
| ★★★★ | Difficult | |
| ★★★★★ | Very difficult |
Specify the resistance of one lamp and the total resistance. Both lamps are similar.
Tip: For lamps in series the total resistance is the sum of resistances.
Answer: Total resistance is $18\ \Omega$; the resistance of one lamp is $9{,}0\ \Omega$