There are two 25s in 50 and there are four 25s in 100. If you remember this, it helps you count in steps of 25. Look at this number line. With every two steps of 25 there is a bigger step of 50.

This number line shows how this works for decimals between 1.6 and 1.7. A place value chart can be really helpful when rounding decimals. They help you see the value of each digit within a number.