You had 1.5 volumes of 1.000 SG water, and (assuming you have 5 gallons of wort) 3.5 volumes of 1.090. The math is:
total SG = (volume 1 * SG + volume 2 * SG) / total volume.
in your case, it's
(1.5 * 1.000 + 3.5 * 1.090) / 5
that gives you 1.063 or so.
Believe it or not, asphalt does not come with one specific gravity
If you are not growing your own 6th generation barley and hops, you're not *really* homebrewing.
Fermenting: Real Ale, Extract Lager (with WLP830), India Pale Ale
Drinking: Pale Ale from the keg