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We can wrte down 2 equations for this then use substitution to solve it.

$6a=8c$

$\frac{3}{4}a+280=c+\frac{1}{4}a$

$\frac{3}{4}a=c$

$c+280=c+\frac{1}{4}a$

$\frac{1}{4}a=280$

$a=1120$

$\frac{3}{4}(1120)=c$

$c=840$

Let's double check:

$6(1120)=8(840)$

$6720=6720$

$\frac{3}{4}(1120)+280=840+\frac{1}{4}(1120)$

$840+280=840+280$

$1120=1120$

So both equations are true, therefore, Andrew has 1120 stamps and Caden has 840 stamps.