What is the value of the third resistor?

[samfulton]

A three-branch parallel circuit has a total resistance of 385.83 ohms. Two of the resistors are 680 ohms and 1500 ohms. What is the value of the third resistor?
Your Answer: 2180 ohms
Why is this wrong

[kwegley]

A three-branch parallel circuit has a total resistance of 385.83 ohms. Two of the resistors are 680 ohms and 1500 ohms. What is the value of the third resistor?
Your Answer: 2180 ohms
Why is this wrong

Let x = third resistor
1/R1 + 1/R2 + 1/R3 = 1/Rtotal
Rearrange the equation
1/Rtotal - 1/R1 - 1/R2 = 1/R3
1/358.83 - 1/680 - 1/1500 = 1/x
When solved x = 2200 ohm

[cbmcbride]

Let x = third resistor
1/R1 + 1/R2 + 1/R3 = 1/Rtotal
Rearrange the equation
1/Rtotal - 1/R1 - 1/R2 = 1/R3
1/358.83 - 1/680 - 1/1500 = 1/x
When solved x = 2200 ohm

I believe this is incorrect, can you actually demonstrate your calculations!