Determine the number of cement blocks needed to put up a wall measuring 20 meters in length and 1.5 meters in height. Each block is 12 cm x 18 cm.

calculate area of wall

20 x 1.5 = 30 square meters

now calculate area of 1 block

12 x 18 = 216 square centimeters

convert cm to meters 1 cm^2 = 0.0001m^2

so 216 x 0.0001 = 0.0216 sq meter for 1 block

divide area of wall by area of block

30/0.0216 = 1388.88 so 1389 blocks are required.