Question

20/100=5/j

Answer 1

You've posted about 15 identical problems that just need you to cross-multiply. Is there something in the technique that is still confusing?

Answer 2

I dont think she is learning anything...

Answer 3

Just like before
\[ \frac{5}{j} = \frac{4 \times 5}{4 \times j} = \frac{20}{100} \]

@moneybird: I urge you not to give just answers

Answer 4

Anyway, now that you have...
@rael.. , you can see that
4j = 100
hence
j = 100/4 = 25.