Ben's new car cost twice as much as his old car. He sold his old car for $1800 and used that to pay off some of the cost of his new car. He still owes another $2000 on his new car. How much did his old car cost, in dollars? Give the rationale and then the answer.
Let's think step by step. His new car costs 1800 + 2000 = 3800 dollars. His old car cost 3800 / 2 = 1900 dollars.. The answer is: 1900.

A housewife goes to the market. She spent 2/3 of her $150. How much does she have left? Give the rationale and then the answer.
Let's think step by step. The housewife has 3 / 3 - 2 / 3 = 1 / 3 of $150 left. This means she has $150 * (1 / 3) = $50 left.. The answer is: 50.

There are 4 more Muscovy ducks than Cayugas, and 3 more than twice as many Cayugas as Khaki Campbells. If there are 90 ducks total, how many Muscovies are there? Give the rationale and then the answer.
Let's think step by step. Let x represent the number of Khaki Campbell ducks. Cayugas:3 + 2x. Muscovies:4 + (3 + 2x) = 7 + 2x. Total: x + 3 + 2x + 7 + 2x = 90. 5x + 10 = 90. 5x = 80. x = 16. Muscovy: 7 + 2(16) = 39.. The answer is: 39.