He now buys 4 / 2 = 2 coffees. The new price is 2 * .5 = $1 more than the old price. So he now pays 2 + 1 = $3 per coffee. So he spends 3 * 2 = $6. He used to spend 4 * 2 = $8. So he saves 8 - 6 = $2 per day.
This justifies what answer for what question? Q & A:
John used to buy 4 coffees a day for $2 each. They raised the price by 50% so he cut the number of coffees he drinks in half. How much money does he save per day compared to what he used to spend?
2