1.3*3=3.90
3.90*35=136.50
136.50 is all you need to provide at least three of the most expensive items.
You still have 13.50 left over
<span>In a Metro there are:
20 rows of seats
18 seats in a row
and $5 per ticket
Using commutative property of Multiplication:
Therefore.
=> (20 x 18) x 5 = 5 x (18 x 20)
=> 360 x 5 = 5 x 360
=> 1800 = 1800
Therefore, they have $1800 income.</span>
Answer: (D) 
Explanation:

Just a note on writing down these expressions: I recommend using parentheses whenever you can to avoid misinterpretation. An expression 1/x-1/y / 1/x+1/y could be interpreted by someone as 1/x-(1/y / 1/x)+1/y, which is a different thing.
2/18
3/27
4/36
5/45
6/54
Take any number let's say 9
Then 9/9 = 1
Or 9/9 = 1/1
Then if you multiply the bottom half of those by 9, you get
9/9 becomes 9/81
and
1/1 becomes 1/9
So to generate any fraction equivalent to 1/9
take any number and write it as one, i.e. 5/5 = 1 (or 1/1)
and then multiply the bottom part by 9 = 5/45