Thanks, but do you mind to explain the units part? How come it became 4/20 and 4/5 part. Which leads to 25 units. Abit confuse on that

When he reduce spending by 20%, the saving increased by 80%,
That mean the spending amount, 20% is the same amount that cause saving to increased 80%.

20% = ⅕
80% = ⅘

For the same amount, make numerator to the same which is 4/20 and 4/5.

If you draw model, spending = 20 unit and saving = 5 unit.
20% of 20u = 4u
80% of 5u = 4u