下图显示了空杯时的质量，以及在杯中放入物体 A、B 和 C 的不同组合时的质量。

1. Read the mass of the empty cup directly from the first bar on the graph

From the graph, the height of the first bar (empty cup) is $70\text{ g}$.

1. Find the combined mass of each pair of objects by subtracting the mass of the empty cup.

Mass of A + B = $280 - 70 = 210$ g
Mass of A + C = $360 - 70 = 290$ g
Mass of B + C = $320 - 70 = 250$ g

2. Find the total mass of A, B, and C, and then find the mass of A.

Sum of the three pairs: $2(A + B + C) = 210 + 290 + 250 = 750$ g
Total mass of A + B + C = $750 \div 2 = 375$ g
Mass of A = Total mass - (Mass of B + C) = $375 - 250 = 125$ g

1. Find the mass of each pair of objects by subtracting the mass of the empty cup.

Mass of A + B = $280 - 70 = 210\text{ g}$
Mass of A + C = $360 - 70 = 290\text{ g}$
Mass of B + C = $320 - 70 = 250\text{ g}$

2. Sum the masses of the three pairs to find the total mass of A, B, and C.

Sum of the three pairs = $210 + 290 + 250 = 750\text{ g}$
Since each object is counted twice in this sum:
$2 \times (\text{Mass of A + B + C}) = 750\text{ g}$
$\text{Total mass of A, B and C} = 750 \div 2 = 375\text{ g}$

3. Calculate the average mass of the three objects.

$\text{Average mass} = \frac{375}{3} = 125\text{ g}$