Answer:
We can increase the output with the same cost increasing the relation work/capital factor relation.
We can keep the same output reducing the cost increasing the relation work/capital factor relation.
Explanation:
The actual situation is:
Work units: 100 units
Work cost: $10 per unit
Marginal productivity of work: $3 per unit
Capital units: 50 units
Capital cost: $21 per unit
Marginal productivity of capital: $5 per unit
For each dollar that we use to increase the work factor, we get 3/10=$0.30 more output.
For each dollar that we use to increase the capital factor, we get 5/21=$0.24 more output.
This values are negative for each dollar of factor of production that is decreased.
With these calculations, we can estimate that the output will grow if we increased the proportion of the work factor in place of the capital factor.
We can measure the cost for two different combinations with the same output.
One is the actual combination, with cost:

If we have 5 more work hours (output grows 5*3=$15), and reduced the capital by 3 units (output is reduced (-3)*5=-$15), we have the same output and its cost is:

We have proved that if we increase the work/capital relation, we can have the same output with less cost.