Answer :

8% of solution of boric acid = 640 litres

Let x litres of 2% boric acid solution is added

Total mixture = x + 640 litres

The resulting mixture has to be more than 4% but less than 6% boric acid.

∴ 2% of x + 8% of 640 > 4% of (x + 640) and

2% of x + 8% of 640 < 6% of (x + 640)

2% of x + 8% of 640 > 4% of (x + 640)

⇒ 2x + 5120 > 4x + 2560

⇒ 5120 – 2560 > 4x – 2x

⇒ 2560 > 2x

⇒ x < 1280 ….(I)

2% of x + 8% of 640 < 6% of (x + 640)

⇒ 2x + 5120 < 6x + 3840

⇒ 6x – 2x > 5120 – 3840

⇒ 4x > 1280

⇒ x > 320 ……….(II)

From (I) and (II)

320 < x < 1280

⇒ the number of litres of 2% of boric acid solution that has to be added will be more than 320 litres but less than 1280 litres.

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