Sorry I still don't understand can you walk me through it.

If half of them will fail by the 1400-hour mark, then only 50% of them will still be working by that time. You are asked to find the time (the number of hours) by which 90% of the bulbs will still be working.

If only 50% are working by 1400 hours, you cannot possible have

*more* working (coming back to life somehow?) at a

*later* time. You can only have

*fewer* working after the 1400-hour mark.

In other words, I think you've been trying to work with the failure rate: finding the time by which 90% would have

*failed*. This will, of course, be after the 1400-hour mark. But you've been asked to find the

*success* rate: finding the time by which 90% will still be working. (This is useful for advertising, so you can promote your bulbs as having an expected lifetime of T hours "or more!!")