Governor Droop Response (Demand Balancing)
The change in speed from reference is:
$$ -0.15 \mathrm{Hz} \div 60 \mathrm{Hz} = 0.25\% $$
Out of the full 5% range of speed adjustment, this will be:
$$ 0.25\% \div 5\% = 5\% $$
Then, multiply for the result:
$$ 5\% \times 100 \mathrm{MW}= 5 \mathrm{MW}$$