Answer: Peter has 170 stickers.
Step-by-step explanation: Given that Peter has twice as many stickers as Joe, Joe has 40 more stickers than Emily and they have 300 altogether.
We are to find the number of stickers that Peter have.
Let x, y and z represents the number of stickers that Peter, Joe and Emily has respectively.
Then, according to the given information, we have

Substituting the value of x and y from equations (i) and (ii) in equation (iii), we get

From equation (ii), we get

and from equation (i), we get

Thus, Peter has 170 stickers.