90 percent round to the thousandths place

I'm pretty sure your talking about 90% to a decimal if you are it would be 0.900

This is quite simple--I'm surprised you asked in the "high school physics".  Nonetheless, 90% is equal to 90/100 which is just 0.9 in fractional form.  0.9 rounded to the thousandths is still 0.9, however, since the problem specifies 'thousandths' it would be appropriate to report the answer as 0.900.