[tex]\bf \begin{array}{ccll}
\stackrel{cake}{oz}&calories\\
\text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\
7.5&900\\
2.5&x
\end{array}\implies \cfrac{7.5}{2.5}=\cfrac{900}{x}\implies x=\cfrac{2.5\cdot 900}{7.5}[/tex]
so, the 2.5oz cake will be "x" calories, pheeew, she'll be taking in 900 - x less calories.
