Suppose you have eight black and white images taken with a $1$-megapixel camera and one $8$-color image taken by an $8$-megapixel camera. How much hard disk space in total do you need to store these images on your computer?

A black and white image needs $1$ bit per pixel.
Size of one $1$-megapixel black and white image $=1\times 10^6$ bits.
Size of $8$ such images $=8\times 10^6$ bits $=1\ MB$.
An $8$-color image needs $\log_2 8=3$ bits per pixel.
Size of one $8$-megapixel $8$-color image $=8\times 10^6\times 3=24\times 10^6$ bits $=3\ MB$.
Total space $=1\ MB+3\ MB=4\ MB$.