A solenoid has 600 turns in a length of 0.25 m and carries 1.5 A…

Step 1. Turns in each metre. $n = \dfrac{600}{0.25\ \text{m}} = 2400$ turns per metre.

Step 2. The field. $B = 4\pi \times 10^{-7}\ \text{T m/A} \times 2400\ \text{m}^{-1} \times 1.5\ \text{A} = 4.52 \times 10^{-3}\ \text{T} = 4.52\ \text{mT}$.

Step 3. Stretched. $n = \dfrac{600}{0.5\ \text{m}} = 1200$ turns per metre, half as many, so $B = 2.26\ \text{mT}$.