The number density of photons left over from the Big Bang has an energy dependence of the form n(E)=
Eʳ/(eᵉᐟᵗ-1), where E is the energy of the photon and T is the temperature of this relic radiation. (For your information, this temperature has been measured to be about -270 degrees Celsius, but don’t use this information for the questions below.)
In order to see how this number density changes with energy, we need the derivative of n(E) with respect to E keeping T fixed. Evaluate dn/dE at fixed T and enter it below.