Chapter 7 - Confidence Intervals and Sample - 7-1 Confidence Intervals for the Mean When σ Is Known - Exercises 7-1 - Page 380: 24

12

At the 95% confidence, the critical z-value is $z_{\alpha/2}=1.96$ Given $\sigma=0.26,E=0.15$, the margin of error can be expressed as $E=1.96\times\frac{0.26}{\sqrt {n}}=0.15$ which gives the sample size as $n=(\frac{1.96\times0.26}{0.15})^2\approx12$ (round up to the next integer here)