One of the biggest problems is that there are so many different concepts and equations in algebra and in order to be successful you have to understand the entirety of it or you will not be able to figure out which procedure to use. You learned the order of operations years ago and that is one key tool for algebra. This tool allows you to understand which process happens first and so on. PEMDAS is a very valuable tool.
If a child is really confused about decimals, converting decimal numnbers into money is a great way to make things clearer. For example: a child may be asked to say how much bigger 1.3 is than 0.9. If they convert these decimals into money (£1.30 and 90p) they may find that they can do this calculation very quickly in their head, getting the answer 40p which they convert back into the decimal, 0.4. Demonstrating that money maths depends on decimal understanding is also an easy way to prove that decimals are actually useful in real life!