After a 680 hour heat treatment at 1130°c the concentration of a is 2.9 wt% at the 3.4 mm position within metal b. at what temperature (in kelvin) must the diffusion couple need to be heated to produce this same concentration at a 1.8 mm position after 680 h? the preexponential and activation energy values for the diffusion of a in b are 2.7 × 10-5 m2/s and 257 kj/mol, respectively.