A diffusion couple similar to that shown in Figure 5.1a is prepared using two hypo- thetical metals A and B. After a 20-h heat treatment at 800°C (and subsequently cooling to room temperature) the concentration of B in A is 2.5 wt% at the 5.0-mm position within metal A. If another heat treatment is con- ducted on an identical diffusion couple, only at 1000°C for 20 h, at what position will the composition be 2.5 wt% B? Assume that the preexponential and activation energy for the diffusion coefficient are 1.5 x 10-4 m2/s and 125,000 J/mol, respectively.