The following steps can be used to calculate the recombination frequency is-

$\mathrm{R}\mathrm{F}=\left(\frac{\mathrm{N}\mathrm{R}}{\mathrm{T}\mathrm{P}}\right)\times 100$ map units where RF is recombinant frequency, NR- number of recombinants, TP- total number of progeny. The total number of recombinants, in this case, is $115 + \mathrm{105 }= 220$ and the total number of progeny is $900+115+880+105=2000$. So the recombination frequency is $\frac{220}{2000}\times 100=11$ map units.