Based on the graph, an accurate prediction of the average sodium concentration at 210 minutes for athletes who drank the sports drink would be expressed numerically as 140mmol/L (It could be any number in the interval between 135 and 145mmol/L).
Sodium is an alkali metal, It is located in the extra-cellular cation (90% extra-cellular) there are very few in the cell. (its normal value is between 135 and 145mmol/L in the blood)
A sports drink is a drink which helps athletes replace water, electrolytes, and energy lost during sports. So in the graph, it is logical to see that the athlete restore his sodium level after drinking.