An athlete eats $55 \mathrm{~g}$ of protein per day while training. How much protein will she eat during 21 days of training? Write your answer in kilograms.

Step 1 ：The problem is asking for the total amount of protein the athlete will consume over a period of 21 days. This can be calculated by multiplying the daily intake of protein by the number of days.

Step 2 ：Let's denote the daily protein intake as \(P\) and the number of training days as \(D\). In this case, \(P = 55\) grams and \(D = 21\) days.

Step 3 ：The total protein intake in grams can be calculated as \(P \times D\). Substituting the given values, we get \(55 \times 21 = 1155\) grams.

Step 4 ：The result should be converted from grams to kilograms by dividing by 1000, since 1 kilogram is equal to 1000 grams. So, the total protein intake in kilograms is \(\frac{1155}{1000} = 1.155\) kilograms.

Step 5 ：Final Answer: The athlete will eat \(\boxed{1.155}\) kilograms of protein during 21 days of training.