How to query order to asc (or desc) when column format is varchar-Collection of common programming errors

I have a table product(ID, name, quanlity) with quanlity is varchar(55)

With my data

1 | Iphone 3  | 7
2 | Iphone 3S | 8
3 | Iphone 4  | 79
4 | Iphone 4S | 9
5 | Iphone 5  | 10

And my query:

Select * From product order by quanlity DESC

But result can’t order quanlity, because quanlity is varchar, How to query sort exactly

    4 | Iphone 4S | 9
    2 | Iphone 3S | 8
    3 | Iphone 4  | 79
    1 | Iphone 3  | 7    
    5 | Iphone 5  | 10