The local hardware store wants to increase their inventory. last year 15 electric screwdrivers cost them $300. at the same cost, how much would 45 screwdrivers cost them this year?

Method 1

45/15 = 3
45 is 3 times 15, so the price is 3 times as much.
3 * $300 = $900

Method 2 using unit cost

15 screwdrivers for $300 means
$300/15 = $20/screwdriver
The unit cost is $20 per screwdriver.
For 45 screwdrivers, the cost is
$20/screwdriver * 45 screwdrivers = $900