Your height: 5'1" to 5'7" (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.
15 speed: perfect to go everywhere including going up very steep hills. There are 3 front chain rings and 5 rear gears. - Derailleur:
Tires 26 x 1.95: wide tires, deep thread, all terrain.