Final answer:
The down payment for the computer store is $540,000.
Step-by-step explanation:
To find the down payment, we need to first calculate the total mortgage amount. The buyer made a 30% down payment, which means they paid 30% of the selling price upfront. So the remaining mortgage amount is 100% - 30% = 70% of $1,800,000. The mortgage amount is 0.7 * $1,800,000 = $1,260,000. Therefore, the down payment is $1,800,000 - $1,260,000 = $540,000.