How can I teach a $7^\text{th}$ grade student the concept of ratio and proportion with the use of teaching aids ( such as models or Charts) . One of my friends asked me this question.
One possible solution could be found using of spring of length length 5 units (,say) and a scale (marked with numbers from $1$ to $10$). I colour the spring to the length of 3 units . Then I stretch the whole spring upto $10(= 5 \times 2)$ units and see the coloured portion stretches upto $6 (=3 \times 2)$ units.
Is there any other easier way (of course by using appropriate teaching aids) to grow the concept of ratio and proportion ?
Thanks in advance for your time.