First, you need to put them in order
53kg, 55kg, 61kg, 61kg, 76kg, 91kg, 98kg, 105kg, 120kg
For mean, you add them all up and divide by the amount of numbers (9)
720/9 = 80
For median, you find the middle number (76kg)
For mode, you find the number that appears the most (61kg)
Mean: 80
Median: 76
Mode: 61