First, translate the question into math.
Here's what I mean by that:
#stackrel(63) overbrace("60") " " stackrel(=) overbrace("is") " " stackrel(90%) overbrace("90%") " " stackrel(xx) overbrace("of") " " stackrel(x) overbrace("what number") "?"#
Now copy that equation and solve for #x#:
#63=90%xxx#
#63=0.9xxx#
#63=9/10xxx#
#63color(blue)(xx10)=9/10xxxcolor(blue)(xx10)#
#63color(blue)(xx10)=9/color(red)cancelcolor(black)10xxxcolor(blue)(xxcolor(red)cancelcolor(blue)10)#
#63color(blue)(xx10)=9xxx#
#630=9xxx#
#color(blue)(color(black)630/9)=color(blue)(color(black)(9xxx)/9)#
#color(blue)(color(black)630/9)=color(blue)(color(black)(color(red)cancelcolor(black)9xxx)/color(red)cancelcolor(blue)9)#
#color(blue)(color(black)630/9)=x#
#70=x#
That's the answer. Hope this helped!