再次求救!! Prep1 PS#135

Machine A produces pencils at a constant rate of 9,000 pencils per hour, and machine B produces pencils at a constant rate of 7,000 pencils per hour.  If the two machines together must produce 100,000 pencils and if each machine can operate for at most 8 hours, what is the least amount of time, in hours, that machine B must operate?

(A) 4

why the answer is (A)?

Thank you guys a lot

9000a+7000b=100,000

每个机器最多工作8小时，所以当a=8时，b的工作时间用的最少，代入可得b=4

ohoh~gocha!!

I thought machine A and B are working "at the same time" so that I got a wrong answer.

thank you

(*^__^*) 嘻嘻……