Answer:
The new length of the garden is 80 ft, the new width is 40 ft, and the total area of the new garden is 3200 ft². The area of the new garden will be 8 times larger than the current one. Jane should make her garden 4 times the current length.
Step-by-step explanation:
Juan's garden is 40 ft in length, he wishes to make the length 2 times longer:

Juan's garden is 10 ft in width, he wishes to make the width 4 times longer:

With a length of 80 and width of 40, the total area of the garden is

Since Juan wanted to make the length 2 times longer and the width 4 times longer, the total area of the garden will be

larger than the current garden.
Jane wants to make her garden 20 times the size. Since she's making her garden 5 times longer in width, she needs to make the length of her garden

4 times longer.