Five 100 Ω resistors are connected in parallel. If one resistor is removed, the total resistance is

• 25 Ω
• 500 Ω
• 100 Ω
• 20 Ω

1/R = 1/R1 + 1/R2 + 1/R3 + 1/R4

1/R = 1/100 + 1/100 + 100 + 1/100

1/R = 4/100

1/R = 1/25

Therefore R = 25 ohm.