[hsarfars]

QUESTION:

The absorbance  in a cell with a 1 cm path length, of a solution containing
NAD+ and NADH is 0.21 at a wavelength of 340 nm and 0.85 at 260 nm. The
molar extinction coefficient of both NAD+ and NADH at 260 nm is
1.8 x10^4 mol-1 dm^3 cm-1 , while the value at 340 nm for NADH is
6.22 x103 mol-1 dm^3 Cm-1. NAD+ does not absorb radiation of wavelength 340
nm. Calculate the concentrations of NAD+ and NADH in the solution

So i understand that i have to apply the beer lambert law for this question but im not sure on how to do so, can anyone give me some tips please?. Thanks in advance

[Borek]

Start with NADH, it is just plug and chug at one of the wavelengths given. At the other wavelength absorbance is a sum of both absorbances.

[Babcock_Hall]

In general you must use two equations in two unknowns to solve this sort of problem.  In this case the absorbance of one compound (NAD) is zero at one wavelength, which allows the problem to be solved in a simpler manner.