Find the missing group of letters in the following series:

BC, FGH, LMNO, _____

The value of the expression

$${1 \over {1 + \log _u^{vw}}} + {1 \over {1 + \log _v^{wu}}} + {1 \over {1 + \log _w^{uv}}}$$ is

A contract is to be completed in 52 days and 125 identical robots were employed, each operational for 7 hours a day. After 39 days, five-seventh of the work was completed. How many additional robots would be required to complete the work on time, if each robot is now operational for 8 hours a day?

Forty students watched films A, B and C over a week. Each student watched either only one film or all three. Thirteen students watched film A, sixteen students watched film B and nineteen students watched film C. How many students watched all three films?