Find the atomic mass of an element whose two isotopic forms with masses 35 units and 37 units and present in 75% and 25% in nature respectively

For the element, the average atomic mass is calculated based on availability of its isotopes.  For Chlorine two isotopes are present. They are 

Cl – 35 and Cl – 37.

${\text{I}}^{\text{st}}\text{Isotope}=\frac{35\times 75}{100}=26.25$

${\text{II}}^{\text{nd}}\text{Isotope}=\frac{37\times 25}{100}=9.25$

Average atomic mass of Cl  $=26.25+9.25=35.5$