Cassie's book has 325 pages she read 155 pages yesterday and the rest today write an equation to express how many she read today

So, first she read 155 pages so you would subtract;

325 pages - 155 pages.

325-155 equals to 170.

So you have 170 pages left to read for today.

So it can be 325 - 155 = x,

Or, you can just put 325 - 155 = 170