a = amount invested at 2%
how much is 2% of "a"? (2/100) * "a", namely 0.02a.
b = amount invested at 7%
how much is 7% of "b"? (7/100) * "b", namely 0.07b.
We know that fund A is less than fund B by 8000, that is a = b - 8000.
we also know that the yielded amount in interest is $1730, so if we simply add their interest, that'd be 0.02a + 0.07b = 1730.
[tex]\boxed{a = b - 8000} \\\\\\ 0.02a~~ + ~~0.07b~~ = ~~1730\implies 0.02(\stackrel{a}{b-8000})~~ + ~~0.07b~~ = ~~1730 \\\\\\ 0.02b-160+0.07b=1730\implies 0.02b+0.07b=1890\implies 0.09b=1890 \\\\\\ b=\cfrac{1890}{0.09}\implies \blacksquare~~ b=21000 ~~\blacksquare[/tex]
