she has 47 total
20 are from Europe
some are from Africa,
and some from Asia
how many from Africa? hmm let's say "a"
how many from Asia? hmm let's say "b"
we know that, she has twice as many "a" as she has "b" stamps,
so, whatever "b" is, a = 2 * b, or a = 2b
so.. let us add them together
\(\bf \begin{array}{llllllll}
20&+&a&+&b&=&47\\
\uparrow &&\uparrow &&\uparrow \\
europe&&africa&&asia\\
\downarrow &&\downarrow &&\downarrow \\
20&+&2b&+&b&=&47
\end{array}\)
solve for "b", to see how many from Asia she has
what's "a"? well, a = 2b , that's how many African ones she has :)
