chemistry help please!!!

Mass spectrometry can be used to identify the chloroauric acid. Naturally occurring chlorine
exists as two isotopes 35Cl (75% abundance) and 37Cl (25% abundance). 197Au is 100%
abundant.
(c) Calculate the m/z values and intensities of the molecular ion peaks for the AuCl4− ion as
a percentage of the total. Use integer masses in the calculation.

I understand how to calculate the m/z values however I am unsure how to calculate the molecular ion peaks, for example for a AuCl4-, the m/z value is 339 so don't you have to do 0.75 cubed times 0.25

This is to do with the fact that there are various possibilities for the Cl isotopes to be present in the molecular ion. For example all could be Cl 35 or all Cl 37, or various combinations of these. You then have to work out the intensities I.e. the percentage of each molecular ion in the total- which will be governed by the abundance of each Cl isotope

thank you for your reply!
yes i tried that but for the mass of 339 i did 197+35+35+35+37 therefore the m/z value is 0.75 cubed times 0.25 but on the mark scheme they multiplied that by 4 and i am unsure why?