Answer
$t_{2}$ = 135h
Work Step by Step
Using Equation 5.6b (page 147 of this book):
${\frac{{x^{2}}}{Dt}}$ = constant
Given:
$x_{1}$ = 2.00 mm
$x_{2}$ = 6.00 mm
D = constant, since temperature is constant
To simplify,
$\frac{x_{1}^{2}}{t_{1}}$ = $\frac{x_{2}^{2}}{t_{2}}$
Substituting the given values:
$\frac{(2.00 mm)^{2}}{15h}$ = $\frac{(6.00 mm)^{2}}{t_{2}}$
$t_{2}$ = $\frac{{(6.00 mm)^{2} \times 15h}}{(2.00 mm)^{2}}$
$t_{2}$ = 135h
