Answer:
Tom, Jerry, Martha (TJM)
Tom, Jerry, Gwen (TJG)
Tom, Anne, Dan, (TAD)
Tom, Anne, Martha, (TAM)
Tom, Anne, Gwen (TAG)
Tom, Connie, Dan (TCD)
Tom, Connie, Martha (TCM)
Tom, Connie, Gwen (TCG)
Sally, Jerry, Dan (SJD)
Sally, Jerry, Martha (SJM)
Sally, Jerry, Gwen (SJG)
Sally, Anne, Dan (SAD)
Sally, Anne, Gwen (SAG)
Sally, Connie, Dan (SCD)
Sally, Connie, Martha (SCM)
Sally, Connie, Gwen (SCG)
Bill, Jerry, Dan (BJD)
Bill, Jerry, Martha (BJM)
Bill, Jerry, Gwen (BJG)
Bill, Anne, Dan (BAD)
Bill, Anne, Martha (BAM)
Bill, Anne, Gwen (BAG)
Bill, Connie, Dan (BCD)
Bill, Connie, Martha (BCM)
Bill, Connie, Gwen (BCG)
Explanation:
Data:
Grade 10: Tom (T), Sally (S), Bill (B)
Grade 11: Jerry (J), Anne (A), Connie (C)
Grade 12: Dan (D), Martha (M), Gwen (G)
We need to form a three-member committee with one person from each grade using all possible combinations. We are already given 2 possible combinations: Tom, Jerry, Dan (TJD) and Sally, Anne, Martha (SAM).
We just need to form the other combinations, one member for each grade (repeated combinations are not allowed):
Combinations:
Tom, Jerry, Martha (TJM)
Tom, Jerry, Gwen (TJG)
Tom, Anne, Dan, (TAD)
Tom, Anne, Martha, (TAM)
Tom, Anne, Gwen (TAG)
Tom, Connie, Dan (TCD)
Tom, Connie, Martha (TCM)
Tom, Connie, Gwen (TCG)
Sally, Jerry, Dan (SJD)
Sally, Jerry, Martha (SJM)
Sally, Jerry, Gwen (SJG)
Sally, Anne, Dan (SAD)
Sally, Anne, Gwen (SAG)
Sally, Connie, Dan (SCD)
Sally, Connie, Martha (SCM)
Sally, Connie, Gwen (SCG)
Bill, Jerry, Dan (BJD)
Bill, Jerry, Martha (BJM)
Bill, Jerry, Gwen (BJG)
Bill, Anne, Dan (BAD)
Bill, Anne, Martha (BAM)
Bill, Anne, Gwen (BAG)
Bill, Connie, Dan (BCD)
Bill, Connie, Martha (BCM)
Bill, Connie, Gwen (BCG)