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I need help understanding the following civil engineering mining question please.

Problem 12.3) The project is in a low-silica marble formation. Company drilling experience in this area indicates that compressed-air rotary-percussion drills with 3-in. button bits can average 2.2 ft of penetration per minute. Assume that it takes 3 min to change a bit. Drilling efficiency is typically a 45-min hour. These drills use 10-ft drill steel. The best blasting pattern is 8 ft x 9 ft with 1.5 ft of subdrilling. The average excavation depth is 12 ft. How many drills will you recommend for use on the job?

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