Answer:
170
Explanation:
The given relations can be used to write and solve an equation for the number of stickers Peter has.
Setup
Let p represent the number of stickers Peter has. That is twice as many as Joe, so Joe has (p/2) stickers. Joe has 40 more stickers than Emily, so the number of stickers Emily has is (p/2 -40).
The total number of stickers is 300:
p +p/2 +(p/2 -40) = 300
Solution
2p = 340 . . . . . . . . . . . . . . add 40, collect terms
p = 170 . . . . . . . . . . . divide by 2
Peter has 170 stickers.
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Additional comment
Joe has 170/2 = 85 stickers. Emily has 85-40 = 45 stickers.
We could write three equations in three unknowns. Solving those using substitution would result in substantially the same equation that we have above. Or, such a system of equations could be solved using a calculator's matrix functions, as in the attachment.
p +j +e = 300
p -2j +0e = 0
0p +j -e = 40