Thallium consists of 29.5% ti-203 and 70.5% ti-205. which is the relative atomic mass of thallium

Relative atomic mass for two isotopes can be calculated as follows

Isotope                        atomic mass                     abundance
²⁰³Tl                                 203                                    29.5%
²⁰⁵Tl                                 205                                    70.5%

Relative atomic mass  
               = (mass of 1 isotope x % abundance) + (mass of 2 isotope x % abundance)
               = (203 x ) + (205 x )
               = 204.41 amu

Hence, relative atomic mass of Tl is 204.41amu