The length of the rectangular bed is 3 m and the width is 90 cm, find the perimeter of the bed.

According to the condition of the problem, the length of the rectangular bed is 3 m, and the width of this bed is 90 cm.
Let’s express the width of this bed in meters. Since there are 100 centimeters in one meter, the width of this bed in meters will be:
90/100 = 9/10 = 0.9 m.
We find the perimeter p of a given rectangular bed as the doubled sum of the length and width of this bed:
p = 2 * (3 + 0.9) = 2 * 3.9 = 7.8 m.
Answer: the perimeter of this bed is 7.8 m.