Concept: The question tests on the area of Rates(Time and Work)

Solution:

The machine producing nuts takes 2000/400 = 5 minutes for every 2000 bolts (assume it as 1 phase)

To produce 12,000 nuts, it will require 12000/2000 = 6 phases of 2000 bolts each = 6 * 5 = 30 minutes

Number of stops (2000 _ 2000 _ 2000 _ 2000 _ 2000 _ 2000) = 5. Stoppage time = 15 * 5 = 75 minutes.

Total Time here = 30 + 75 = 105 minutes


The machine producing nuts takes 3000/300 = 10 minutes for every 3000 bolts

To produce 12,000 bolts, it will require 12000/3000 = 4 phases of 3000 bolts = 4 * 10 = 40 minutes

Number of stops (3000 _ 3000 _ 3000 _ 3000 ) = 3. Stoppage time = 15 * 3 = 45 minutes.

Total time here = 40 + 45 = 85 minutes


Minimum time to make 12,000 pairs = Max (105, 85) = 105 minutes (option c)