Grape Juice concentrate 3:1 is reconstituted by mixing one volume of
concentrate with three volumes of water. Consumers indicated that this
tastes artical; therefore, manufacturing is producing a new 4:1
concretrate. Before packing, this will be blended with fresh juice to
produce a 3:1 concentrate. If you want to produce 1000kg of the new
product, how much 4:1 concentrate must be prepared, and how much grape
juice muste be kept in reserve for dilution purposes?

Hello.

This same problem, using "orange juice," appears on this web site:
Food Plant Engineering System, Chapter 2
http://webhome.crk.umn.edu/~crobbert/Engineering/ch2_6.asp

According to the solution on that site, 800 kg of 4:1 concentrate
would be prepared while 200 kg of juice would be added for dilution
purposes.

"You will add 200 kg to 800 kg concentrate to give 1000 kg 1:3 concentrate."

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search strategy:
"produce a 3:1 concentrate"

I hope this helps.