A fence will be built to enclose a 100-m by 70-m field. It will need a post at each corner and one every 5 m. How many posts are needed?

The answer is 68.

Field had shape of rectangle since has difference in length and width. The perimeter of rectangle can be expressed as:

P = 2 * (a + b)

If:

a = 100 m

b = 70 m

Then

P = 2 * (100 m + 70 m)

P = 2 * 170 m

P = 340 m.

If posts are on every 5 m, the number of needed posts is:

340 m : 5 m = 68.

Thus, 68 posts are needed for the fence.