x+3y=900

x+6z=900

3x=900

X=300, y=200 and z= 100 are obtained.

Let me put it this way. From the third picture, we can calculate that the weight of apples is 900/3 = 300g. Combining the first picture and the second picture, we can know that three peaches = 600g and six strawberries = 600g, so a peach is 600/3 = 200g and a strawberry is 600/6 = 100g. Can you understand this?