To write a word problem that involves a proportional relationship:
Sam bought 4kg of apples for $12. How many kilograms of apples, he can buy for $30?
Sam bought 4kg of apples for $12.
So, cost of 1 kg of apples is,

Let x be the number of kg apples.
Therefore, He can buy 3x kg apples for $30.
So,

Therefore, He can buy 10 kg apples for $30.