Answer: 20 = 5 × n ; n = 4
How do we know?
If the blue jar has 20 marbles, and the blue jar has 5 times as many as the red jar, and n is how many marbles are in the red jar then we can use this equation:
n × 5 = 20
So for the first portion of your question the answer would be: 20 = 5 × n
Now we have to solve for n:
20/5 = n
4 = n
So the answer to your second part would be: n = 4
