Your height: 4'11" to 5'5" (150 to 164cm) approximatly (?).
Frame size = 20in 51cm
Comments:
Mountain Bikes: they are sturdy and designed to go everywhere including off-road. Good for rough riding. In general heavy and with deep thread tires slow on paved roads. Not good city bikes unless narrower and slick tires are installed.
21 speed: perfect to go everywhere including going up very steep hills. There are 3 front chain rings and 7 rear gears. - Derailleur: Shimano Altus
Tires 26 x 1.95: wide tires, deep thread, all terrain.