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!