5 dozen peaches is 5*12=<<5*12=60>>60 peaches.
Let "x" be the number of peaches put into the knapsack.
Thus, the total number of peaches is x+(2*x)+(2*x)=60.
Simplifying the expression, we see that 5*x=60.
Dividing each side of the equation by 5, we see that x=12 peaches,
#### 12