Marco and his dad went strawberry picking. Together they collected strawberries that weighed 36 pounds. On the way back Marco ' dad lost 8 pounds of strawberries. Marco's strawberries now weighed 12 pounds. How much did his dad's strawberries weigh now?
We have:

strawberries1 kg * 2.5 price = $

strawberries2 kg * 2.5 price = $

Now we can introduce an intermediate variable y - the number of kilograms of strawberries sold by the father:

y kg * 2.5 price = $

Now we can write two equations and solve them:

y + 0.4 = 2

y = 2 - 0.4

y = 1.6 kg