What is the atomicity of aluminium phosphate?

An oxide of an element contains $20 \%$ oxygen by weight. Calculate the equivalent weight of the element.

The equivalent weight of a certain trivalent element is $20$. The molecular weight of its oxide is:

The relative abundances of two isotopes of rubidium with atomic masses $85$ and $87$ are $75\%$ and $25\%$ respectively. What is the average atomic mass of rubidium (in $.5$)?

Atomic weight of an element is $x$. The actual mass of one atom of that element is

Naturally occurring boron consists of two isotopes with atomic masses $10.01 \ (I)$ and $11.01 \ (II)$. The average atomic mass of natural boron is $10.81$. Find the percentage abundance of isotopes $(I)$ and $(II)$ respectively.