[small1dawg]

I know this is probably a simple question but its been a while since i have taken chemistry. 
You have two water streams, one with a value of 400 TDS (stream A) and the other with a value of 40 TDS (stream B), that flow into a holding tank and thoroughly mix.  The one water stream that flows out of the holding tank has a TDS of 100 (stream C).
I was thinking it would be 60% stream A and 40% stream B but i'm not sure of the exact way to solve the problem.  Any helpt would be greatly appreciated thanks!!

[amorale]

This is a Material Balance problem which I can try to solve but I would need to know the amount of material flowing in the streams. For example 10L/min in stream A and lets say 100L/min in stream B or if it is a batch process in which lets say maybe 10L of stream A and 100L of stream B were release into the tank and then it was aloud to mix and then release in stream C before adding any new amounts to get in the tank.

Also what are the units of the Total Disolved Solids measurement because there are alot of different units for TDS depending on the measuring equipment.

However I was able to work something out that might help you.
Assumptions: Units of TDS are in mg/L

So if I add 16.7L of stream A to the tank and 83.3L of stream B I get a total volume of 100L inside the tank and I will also assume that at the end of the mixing I will completely empty the tank therefore removing the 100L of solution.
Therefore we have a batch operation and no inventory change in the tank.

16.7L*(400mg/L)= 6680mg
83.3L*(40mg/L)  = 3332mg

so inside the tank we have 100L total and 10,012mg of TDS

10,012mg/100L = 100.12mg/L of TDS
This will become stream C once the tank is drain.

So we can say that in order to mix two streams, A(400mg/L) & B(40mg/L) and get a stream C of 100mg/L.
Stream C needs to be made of about 16.7% Stream A and 83.3% Stream B.

Again this only works if my assumptions about the units of TDS you are referring are in mg/L
Also is like 2am so check for calculation errors lol. Hope this answers your question.