Answer:
The correct quantities are 8 gallons of 70% and 12 gallons of 20%.
Step-by-step explanation:
Boys have to prepare 20 gallons of 40% mixture using 20% and 70% mixtures.
Therefore, lemon in final mixture = 40% of 20 gallons = 8 gallons.
Let number of gallons taken from 20% mixture be x
So, Lemon contribution by this mixture = 20% of x gallons.
[tex]=\frac{x}{5}\thinspace gallons.[/tex]
Now, since the mixture has to be 20 gallons, we have to take (20-x) gallons of 70% mixture.
Lemon contribution by this mixture =70% of (20-x)
[tex]=14-(\frac{7\cdot x}{10})\thinspace gallons[/tex]
But, these both mixture should be added to get the final mixture :
[tex]\implies\frac{x}{5} +14-(\frac{7\cdot x}{10}) =8\\\\\implies (2-7)\cdot \frac{x}{10} = -6\\\\\implies x = 6\times \frac{10}{5}\\\\\implies x = 12\thinspace gallons.[/tex]
Thus, 20% mixture = 12 gallons and 70% mixture = 8 gallons