Answer:
There are 27 boxes are on the shelf at the start of the day.
Explanation:
Let number of boxes are on the shelf at the start of the day is x
According to given situations :
On one day, a store sold 16 boxes of rice
i.e,

Restocked the shelf with 22 boxes,
⇒

But 27 boxes sold
⇒
Restocked with 30 boxes,
⇒
But 15 boxes sold,
⇒

There are now 21 boxes of rice on the shelf.
So,


Therefore,There are 27 boxes are on the shelf at the start of the day.