Can anybody me with number number 20 u

The property used in #20 was called 'the distributive property'.

This property gets rid of the parenthesis and also gets the expression more simple. 

x(y+b)
x*y + x*b              ⇔ Notice the change here? 

Answer would be: Distributive property 

4. 65 because she has 25/40 CDs and Paolo is saying that his amount is equivalent to what she has and 25+40 is 65 so out of the the CDs Paolo has 50 out of 65 of them are rock CDs.  
5. 108 because you multiply 270 by 0.40 because that is how you find percentages and you end up with 108. 
6.10 because like question 5 you turn the percentage into a decimal and since 25 is 2 digits instead of doing 0.02 or 0.025 we get to do 0.25 x 40 and 0.25 x 40 is 10. 
7. 0.86 because 43 out of 50 as a decimal would be 0.86. 
8.
A. 2/3 Because 1/3 of those are sesame so that means 2/3 of those bagels are plain. 
B. 1/3 because in order to turn a ratio into a decimal all you have to do is remove the colon punctuation aka : and put a divider aka / instead and you get 1/3 
C. 8 sesame 16  plain because you take 12+12 (two dozens) and 1/3 of 24 is 8 and 2/3 of 24 is 16.