Answer:
51 rows
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