Add: 5 [0.5k -1] +2 [0.4 - k]. Answer should be in decimal form.

To solve this expression, we can distribute the coefficients to each term and then combine like terms.

5 [0.5k -1] + 2 [0.4 - k]
= 5 * 0.5k - 5 * 1 + 2 * 0.4 - 2 * k
= 2.5k - 5 + 0.8 - 2k

Now, let's combine like terms:

2.5k - 2k - 5 + 0.8
= 0.5k - 4.2

Therefore, the final answer in decimal form is 0.5k - 4.2.