Miraa is making telescopes, each consisting of 2 lenses, 1 tube, and 1 eyepiece. Lenses can be purchased only in packs of 50, tubes only in packs of 10, and eyepieces only in packs of 30. However, half of the lenses in each pack are not usable for telescopes. If all parts are used only for the telescopes, what is the minimum number of lenses Miraa must purchase to make a set of telescopes with no leftover components other than the unusable lenses?
Select:  A) 75  B) 150  C) 3000  D) 600  E) 7,500
I'd use backsolving for this question.   Check C:   3000 lenses = 1500 usable lenses = 750 tubes = 750 eyepieces. We CAN buy 750 tubes and 750 eyepieces, so that not to have leftovers.   Answer: 3000.
<result>C</result>
Miraa is making telescopes, each consisting of 2 lenses, 1 tube, and 1 eyepiece. Lenses can be purchased only in packs of 50, tubes only in packs of 10, and eyepieces only in packs of 30. However, half of the lenses in each pack are not usable for telescopes. If all parts are used only for the telescopes, what is the minimum number of lenses Miraa must purchase to make a set of telescopes with no leftover components other than the unusable lenses?