Translate the sentence from English to math (call "a number" #n#) then solve for #n#:
#stackrel(36) overbrace("36") " " stackrel(+) overbrace("plus") " " stackrel(n) overbrace("a number") " " stackrel(=) overbrace("equals") " " stackrel(59) overbrace("59") "."#
Copy down the equation, then subtract #36# from both sides:
#36+n=59#
#36+ncolor(blue)-color(blue)36=59color(blue)-color(blue)36#
#n+36color(blue)-color(blue)36=59color(blue)-color(blue)36#
#ncolor(red)cancelcolor(black)(+36color(blue)-color(blue)36)=59color(blue)-color(blue)36#
#n=59color(blue)-color(blue)36#
#n=23#
This means that the number that the problem was describing is #23#. Hope this helped!