How many baseballs equal one pineapple?

Let a be an apple, b be a baseball, p be a pineapple and s be a shoe.

Then, from the balance, we model these equations:

p + b = s     (1)

2s = 4a

Divide both sides by 2.

s = 2a

Substitute in (1), we get,

p + b = 2a   (2)

Also,

p = a + b    (3)

Substitute the above in (2), we get,

(a + b) + b = 2a

a + 2b = 2a

Subtract a from both sides.

a = 2b

Substitute in (3), we get,

p = 2b + b = 3b

Hence, 3 baseballs equal one pineapple.