| 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.
Answer: Total resistance is $5{,}1\ \Omega$; the resistance of one lamp is $10\ \Omega$