Question

Christina collects stamps. She has 47 stamps in all. She has 20 stamps from europe. The number of African stamps is 2 times the number of Asian stamps. How many stamps from each continents does she have?

Answer 1

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 :)