what is the percentage of aluminum in aluminum phosphate?

AlPO4

Al ( aluminum) = 26.98 g/mol

PO4 ( polyatomic ion phosphate ) = 94.97 g/mol

Now, just a simple percentage exercise.

26.98 g/mol/94.97 g/mol * 100

= 28%

======Which was already the answer, but now you know how to do this.