A row of tiny red beads is placed at the beginning and at the end of a 20‐centimeter bookmark. A row of the same beads is also placed every 4 millimeters along the bookmark.
How many rows of the beads are used on the bookmark?

51 rows

Step-by-step explanation:

Given

Length of bookmark = 20cm

Distance between beads = 4mm

Required

Number of rows of beads

First, the distance between the rows of beads must be converted to cm

if 1mm = 0.1cm

then

4mm = 4*0.1cm

4mm = 0.4 cm

This means that each row of beads is placed at 0.4 cm mark.

The distance between each row follows an arithmetic progression and it can be solved as follows;

Where (The last term)

(The first term)

(The distance between each row of beads)

n = ?? (number of rows)

Solving for n; we have the following;

becomes

DIvide both sides by 0.4

Add 1 to both sides

Hence, the number of rows of beads is 51