Simple arithmetic operations with observed spectra can be performed by the command OPERATE/SPECTRUM. The input requires two spectra stored in different files. The fundamental operations + - / can be applied in a direct way to these spectra. Correct error propagation has been applied. The resulting spectrum will be stored in a separate file. In addition these arithmetic operation the operation & performs the time-average of two spectra by conserving the count rate of the source. In this case the error of the count rate will decreased according to the statistical error of the two spectra.