Which is the best measure of center to summarize the reading times? Range. The reason: It is the difference between the high and low value.

Median. The reason: It is less influenced by the one very low value.

Mean. The reason: It takes into account all the values.

Mode. The reason: It is the time most people spent reading.

15 liters of pure acid and 120 liters of 10% solution are needed.

Step-by-step explanation:

Let us assume that,

x = liters of pure acid ,

y = liters of 10% acid.

As we need a total of 135 liter of batch, so

 ------------------------------1

As we need to make a batch of 20% acid solution, hence acid content of the mixture of two acids will be same as of the final one, so

 -----------2

Subtracting equation 2 for 1,

Putting this in the equation 1,

Therefore, 15 liters of pure acid and 120 liters of 10% solution are needed.