Your elderly neighbor places a bag of rice on your doorstep each morning. The first day, the bag contained 1 grain of rice, the second day it contained 2 grains of rice, 4 grains of rice the third day, and so on. Each grain of rice weighs 0.04 grams. How many grains of rice will be in the bag on the 21st day?

solution:

→c= a b , that is product of a and b is possible only when when a is m × n matrix and b is n × p matrix.

that is, number of columns of a= number of rows of b.

so, size of matrix , c= a b is , if a = (m × n)matrix

b= (n × p)matrix

then a b = [ (m × n)matrix] ×[   (n × p)matrix ]

→→c= a b= [m × p ] matrix

→→size of product a b will be equal to = number of rows of a × number of columns of b

the first option is the answer.