For Question 16, Page 134 - you can find a very similar question here with two different ways to solve it
example solution
The actual solution can be found in the online solutions PDF
online solutions
Question 20, Page 136, the solution is in the back of the book.
Here is an alternative with a model. I have broken out 50-cents into 20-cent and 30-cent so we can compare the bar for 20-cents.
The sq and triangle concept just boggles my mind. I don't know when to assume for some questions and when not to assume. The gap-and-difference thing also is incomprehensible.
By Sq and Triangle concept, do you mean Patterns involving Square Numbers and Patterns involving Triangle numbers? For both and for Gap and Difference you can search for all three in this forum and see many examples as well as the solutions in the back of the +hinkingMath book. It is a little difficult to provide a general answer that will help you at this point. It would be better to use specific questions you are having problems with. For this if you post the question and your solution that we can see where you are having trouble and give you some specific advise / help.
Finally, with regards to assuming - this is a vague comment as well. When solving problem sums, I almost never assume anything. Instead, I break down the question one sentence at a time gaining information from each one allowing me to formulate a strategy / approach to apply. If I do not immediately know how to solve it, I draw a picture, model, timeline, lines, anything to help me get some clarity and/or to identify (uncover) more information that I could get through inferencing. If I still do not see how to proceed, I do another question and come back to it 10-15 minutes later - hopefully with a different set of eyes to see the question differently.
I know some of my answers are vague, however, it's difficult to give you specific comments when the question is vague too
HELP!!!! I need help for the maths question 4 pg 128 for Chap. 9 Key Constructs and question 16 Pg 134 and question 20 pg 136! (p6 onsponge thinking maths) I don't understand it at all! Pls help. The sq and triangle concept just boggles my mind. I don't know when to assume for some questions and when not to assume. The gap-and-difference thing also is incomphrensible.(Excuse me if i spelled it wrongly) 
