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If you talk about proportion, then it is also tackles about ratio, specifically variation.

You can find the missing term of variation through finding first it's constant of variation.

There are 4 kinds of variation.
Direct, Inverse, Joint and Combined.

You may able to tackle this during grade 9. If you want an advance study, there's no mistake to reaserched it.

Hope it Helps =)

-Total Blacknout here =(
Write the proportion, using a letter to stand for the missing term and then find the cross products by multiplying 20 times x, and 50 times 30. Then divide to find x. Study this step closely, because this is a technique we will use often in algebra.