The bottle of shampoo is half price for 2.10 was 4.20, its a 500 ml bottle of shampoo.

The unit price of the hair shampoo is given in cents per 100 millilitres (mL).

What is the unit price for the hair shampoo when it is on special?

If you would like to know what is the unit price for the hair shampoo, you can calculate this using the following steps:

500 ml ... $2.10

100 ml ... $x = ?

500 * x = 2.10 * 100 / 500

x = 2.10 * 100 / 500

x = $0.42

The unit price for the hair shampoo is $0.42.