You can make an equation.
Let x=oranges
p=plums
n=apples
n= 3p + 5x
So basically we know 1 Apple means Sam will pack 3 plums and 5 oranges
To get 3n which means 3 apples we have to multiply the whole equation by 3
(n= 3p + 5x) x3
3n=9p+15x
So Sam will also put 9 plums and 15 oranges if he puts 3 apples in the box
