Identify

$ identify IMG_4417.JPG
IMG_4417.JPG JPEG 2112x2816 2112x2816+0+0 8-bit DirectClass 2.543MiB 0.000u 0:00.000

Using the command line calculator

Non-interactively:

echo "scale=2; 2112 / 2816" | bc

Interactively:

$ bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
scale=2; 2112 / 2816
.75