Anita sees in her nutrition book that a 7.5-ounce cake has 900 calories how many calories will she save if only takes 2.5-ounce piece

You can set up a proportion. The quick answer is 600

7.5/900 = 2.5/x
x = 900 * 2.5 / 7.5
x = 900 * (1/3)
x = 300

But that is not the answer. The question is what does she save.
900 - 300 = 600

You could get the answer more directly. She does not eat 5 oz cake, so solve for that.

5/x = 7.5/900 Cross multiply
5 * 900 = 7.5 x
4500 = 7.5x Divide by 7.5
4500/7.5 = x 
600 = x