Well to do this you've got to divide 10 by 1 1/4, so you're going to want to turn both into mixed numbers.
10 is 10/1, and 1 1/4 will be 5/4. Now you can divide 10/1 by 5/4.
In order to divide fractions, you have to use the inverse operations to get the answer -- flip it all around.
10/1 stays the same, but the division sign and 5/4 get flipped. This turns the problem into 10/1 * 4/5
10/1 * 4/5 will equal 40/5, and to simplify you just have to divide that. 40 / 5 = 8, so you can fit 8 shakers worth of salt into this box.