I need to write a UDF.

The UDF needs to include $4$ variables:

RQ $($volumetric flowrate$),$ RDO $($ DO of river$),$ SQ$($ volumetric flow of stream$),$ & SDO$($DO of stream$)$

The UDF must check the stream flow against river flow, and use the mixing formula to calculate the downstream DO$.$ If the stream flow is less than $18\%$ the UDF should return the DO of the river alone.

please!