Answer:
58.3%
Step-by-step explanation:
The formula of the compound is given as: 
 CsPO₄ 
Now, the percentage composition of Cs in the compound is = ? 
So; 
To find this parameter, find the molar mass of the compound: 
 Molar mass of CsPO₄ = 132.9 + 31 + 4(16) = 227.9g/mol 
 Percentage composition of Cs= 
 = 58.3%
 = 58.3%