请教国和F2一个很简单又让我迷糊的题

jadore2007

F2 p146  我就简易叙述关键题干了
skilled workers paid $10 per hours
skilled OT 20 hours this week, 12h on specific orders and 8 h for general . OT paid at a rate of time and a quarter


书上答案是specific ot =12h*$10*1.25
这个1.25是怎么来的？
急死了，希望国和帮帮我