L is 2.5% concentrated while H is 7.5% concentrated. How many litres of the "H" must be added to 500 L of "L" to get a mixture which is 6.5% concentrated?

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Suppose h is the amount of H that is needed to be added,

The number of units in 500 L of L + The number of units in h L of H = The number of units in (h+500) of the mixture

`500 * (2.5/100) + h * (7.5/100) = (h+500) * (6.5/100)`

`1250 + 7.5 h = 6.5 h + 3250`

h = 2000 L

The amount of H to be added is 2000 L.

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