A manufacturer has 640 litres of a 8% solution of boric acid. How many litres of a 2% boric add solution be added to it so that the boric acid content in the resulting mixture will be more than 4% but less than 6%?
Let x litres of 2% of boric acid be added to 640 litres of 8% solution of boric acid. Them total mixture = (640+x)litres
Boric acid content in this mixture =(640×8100+x×2100) litres.
∴ 4100×(640+x)<{640×8100+x×2100}<6100×(640+x)
⇒2560+4x<5120+2x<3840+6x
⇒2x<2560 and 4x>1280⇒320<x<1280.