Light is travelling in quartz which has an index of refraction of 1.458. How fast does light travel in quartz?

The definition of the index of refraction is
##n = c / v##
where ##c## is the speed of light in a vacuum (##3 xx 10^ 8## ##m##/##s##)
and ##v## is the speed of light in the material.
See if you can use this to figure the speed of light in quartz since you know the index of refraction.