jmc

Replace a triangle with the letter $a$ and a circle with the letter $b.$ The two given equations become $$\begin{array}{rl}5a+3b& =21\\ 3a+5b& =16\end{array}$$Multiplying the first equation by $3,$ we get $15a+9b=63.$ Multiplying the second equation by $5,$ we get $15a+25b=80.$ Subtracting this last equation from the second-to-last equation to eliminate $a,$ we have $16b=17.$ Multiplying both sides by $\frac{2}{16},$ we get $$\frac{2}{16}\cdot 16b=\frac{2}{16}\cdot 17⟹2b=\frac{17}{8}.$$Thus, two circles equals $\boxed{\frac{17}{8}}.$