Answer: choice A. 40 liters
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Explanation:
x = number of liters of the 50% alcohol solution
If we have x liters of 50% alcohol, then we have 0.50*x liters of pure alcohol. This is added to 0.90*40 = 36 liters of pure alcohol (from the 90% solution).
So far we have 0.50*x + 36. This expression represents the total amount of pure alcohol. We want a 70% solution, so we want 70% of the total 40+x meaning 0.50*x + 36 is to be set equal to 0.70*(40+x) and we solve for x as shown below
0.50*x + 36 = 0.70*(40+x)
0.50*x + 36 = 0.70*(40)+0.70*(x)
0.50*x + 36 = 28+0.70*x
36 - 28 = 0.70*x - 0.50x
8 = 0.20x
0.20x = 8
x = 8/0.20
x = 40
So that is why the answer is choice A. 40 liters
47.60514678
What I did was 28 squared plus b squared equals 38.25 squared and ya......
let's recall that a cube is just a rectangular prism with all equal sides, check picture below.
![\bf \textit{volume of a cube}\\\\ V=s^3~~ \begin{cases} s=&length~of\\ &a~side\\ \cline{1-2} V=&27000 \end{cases}\implies 27000=s^3\implies \sqrt[3]{27000}=s\implies 30=s \\\\[-0.35em] ~\dotfill\\\\ \textit{surface area of a cube}\\\\ SA=6s~~\begin{cases} s=&length~of\\ &a~side\\ \cline{1-2} s=&30 \end{cases}\implies SA=6(30)\implies SA=180](https://tex.z-dn.net/?f=%5Cbf%20%5Ctextit%7Bvolume%20of%20a%20cube%7D%5C%5C%5C%5C%20V%3Ds%5E3~~%20%5Cbegin%7Bcases%7D%20s%3D%26length~of%5C%5C%20%26a~side%5C%5C%20%5Ccline%7B1-2%7D%20V%3D%2627000%20%5Cend%7Bcases%7D%5Cimplies%2027000%3Ds%5E3%5Cimplies%20%5Csqrt%5B3%5D%7B27000%7D%3Ds%5Cimplies%2030%3Ds%20%5C%5C%5C%5C%5B-0.35em%5D%20~%5Cdotfill%5C%5C%5C%5C%20%5Ctextit%7Bsurface%20area%20of%20a%20cube%7D%5C%5C%5C%5C%20SA%3D6s~~%5Cbegin%7Bcases%7D%20s%3D%26length~of%5C%5C%20%26a~side%5C%5C%20%5Ccline%7B1-2%7D%20s%3D%2630%20%5Cend%7Bcases%7D%5Cimplies%20SA%3D6%2830%29%5Cimplies%20SA%3D180)
It will be easier to understand this using a venn diagram :
