__Explanation:__

For laminar flow over a flat plate

**Blausius equation** is:

\(\begin{array}{l} \frac{\delta }{x} = \frac{5}{{{{\left( {{R_{{e_x}}}} \right)}^{\frac{1}{2}}}}}\\ \Rightarrow \frac{\delta }{x} = \frac{5}{{{{\left( {\frac{{\rho Vx}}{\mu }} \right)}^{\frac{1}{2}}}}} \end{array}\)

\(\Rightarrow \frac{\delta }{x} = \frac{{constant}}{{\sqrt x }}\) {∴ δ, v & μ are constant}

\(\begin{array}{l} \Rightarrow \frac{\delta }{{\sqrt x }} = constant\\ \Rightarrow \frac{{{\delta _1}}}{{\sqrt {{x_1}} }} = \frac{{{\delta _2}}}{{\sqrt {{x_2}} }}\\ \Rightarrow {\delta _2} = \sqrt {\frac{{{x_2}}}{{{x_1}}}} {\delta _1}\\ \Rightarrow {\delta _2} = \sqrt {\frac{{0.75}}{{0.25}}} \times 8 \end{array}\)

⇒ δ

_{2} = 13.86 mm