Let x be the saving of one brother and y be the saving of other brother. Their combined saving is, then, x + y = 55. Since one brother has 15 bucks more that the other we have x = y + 15, or y = x +15. We got here two equations system {x + y = 55, x = y + 15}. Inserting the expression for x into the first equation we find the value of y, inserting the latter into the equation for x we find it.
solve(x + y = 55, x = y + 15, x, y)
+259 
