The average duration for the drying of concrete will vary when the quantities of both concrete and water used are not the same.
The average time it takes for concrete to dry is different when the quantities of concrete and water used are not the same.
The question is asking for the average time it takes for concrete to dry when the amounts of concrete and water are not the same. To paraphrase it, we can say: "What is the average drying time for concrete when the amounts of concrete and water used are different?"
