Answer:
28.29 gallons
Step-by-step explanation:
Given that the available amount of 21%-concentrated red dye= 66 gallons.
Let x gallons of 51%-concentrated red dyes to be mixed with 56 gallons of 21%-concentrated red dye to get 30%-concentrated red dyes.
Total amount of the resulting mixture = 66 + x gallons.
Amount of red dyes in the mixture = (21% of 66) + (51% of x)
To get the resulting mixture as 30%-concentrated red dyes, we have
30% of (66+x) = (21% of 66) + (51% of x)
Hence, the amount of 51%-concentration red dyes to be mixed = 28.29 gallons.