1/1*4+1/4*7+1/7*10+.+1/91*94
=1/3 * (1 - 1/4) + 1/3 * (1/4 - 1/7) +...+ 1/3 * (1/91 - 1/94)
=1/3 * (1 - 1/4 + 1/4 - 1/7+...+1/91 - 1/94)
=1/3 * (1 - 1/94)
=31/94