Hello,
1 liter of water in normal conditions has a volume of 1dm^3=(1dm)^3=(10 cm)^3=10^3 cm^3=1000 cm^3
Given:
Bobby put 1/3 of his lawn mowing money into his savings
He uses the remaining 2/5 to buy a video game.
He has $12 left.
To find:
The amount did he have at first.
Solution:
Let x be the initial amount.
Bobby put 1/3 of his lawn mowing money into his savings. So, the remaining amount is

He uses the remaining 2/5 to buy a video game. Then the remaining amount is





It is given that the remaining amount is $12.



Divide both sides by 2.


Therefore, Bobby have $30 at first.
Again, a spreadsheet or suitable calculator makes this short work.
The sum is 184.
_____
The terms being summed are ...
2·3² +3 = 2·9 +3 = 18 +3 = 21
2·4² +3 = 35
2·5² +3 = 53
2·6² +3 = 75
Their total is
21 +35 +53 +75 = 184