3 men can complete a piece of work in 15 days. 6 women can also complete the same work in 15 days. In how many days will 4 men and 4 women together complete the same piece of work?

3 men complete work in 15 days, so 1 man's 1 day work = 1/(3×15) = 1/45. 6 women complete in 15 days, so 1 woman's 1 day work = 1/(6×15) = 1/90. Combined work of 4 men + 4 women in 1 day = 4/45 + 4/90 = 8/90 + 4/90 = 12/90 = 2/15. Days needed = 15/2 = 7.5 days. Since 7.5 is not among the given options, E (Other than those given) is correct.