The first thing I would do is convert that 1.2 kilograms of salt to grams; that would ensure that all the units are the same so we can perform calculations on them.
1.2kg = 1200g
Next, I would find out how many students can do the experiment with 1200g of salt, by finding it how many 25g are in 1200g (by dividing them).
[tex] \frac{1200g}{25g}=48students [/tex]
That means we still have 80 students left to provide salt for (128 - 48).
Therefore, he needs an additional 80 amounts of 25g of salt.
Therefore he needs:
[tex]80students*25g = 2000g = 2kg[/tex]
