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A family consisting of one mother, one father, two daughters [#permalink]
22 Nov 2006, 12:50
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A family consisting of one mother, one father, two daughters and a son is taking a road trip in a sedan. The sedan has two front seats and three back seats. If one of the parents must drive and the two daughters refuse to sit next to each other, how many possible seating arrangements are there?
step 1. on the driver seat, there are 2 possibilities.
step 2: consider the rest of the seats. Categorize them.
Category 1: the other parent on front seat.
back seat: daugthers have to separated. u have 2 choices.
so cat1: 2
Category 2: son at front.
back seat is similar to cat 1. 2
so cat 2: 2.
Category 3: one of the daughters is in the front seat. Remember either daughter could be in the front, so remember to multiply the back seat counts by 2 later.
Back seat: There are a parent, a son, and a daughter. They don't have any restrictions of seating, since a daughter is already seating in the front. so their arrangements is permutation of 3: i.e. 3!
So for Cat 3, there are totally 2*3! Multiply 3! by 2 is because the 2 daughters can exchange their seats.
Then total:
2*(2 + 2 + 2*3!) = 32.
Last edited by tennis_ball on 22 Nov 2006, 21:36, edited 2 times in total.
step 1. on the driver seat, there are 2 possibilities.
step 2: consider the rest of the seats. Categorize them.
Category 1: the other parent on front seat. back seat: daugthers have to separated. u have 2 choices. so cat1: 2
Category 2: son at front. back seat is similar to cat 1. 2 so cat 2: 2.
Category 3: one of the daughters is in the front seat. Remember either daughter could be in the front, so remember to multiply the back seat counts by 2 later.
Back seat: There are a parent, a son, and a daughter. They don't have any restrictions of seating, since a daughter is already seating in the front. so their arrangements is permutation of 3: i.e. 3!
So for Cat 3, there are totally 2*3! Multiply 3! by 2 is because the 2 daughters can exchange their seats.
Then total:
2*(2 + 2 + 2*3!) = 32.
Sorry but little confuse here.
Father=F
Mother=M
Son=S
Daughter1=D1
Daughter2=D2
Cat 1: F-M(D1-S-D2) --> times 2 since you can switch F and M = Total 2
Cat 2: F-S(D1-M-D2) --> times 2 since you can switch F and M = Total 2
Cat 3a: F-D1(3!) --> times 2 since you can switch D1 and D2 = Total 12
Cat 3b: M-D1(3!) --> times 2 since you can switch D1 and D2 = Total 12
Three cases
Case 1 Front seats by parents and back seats by daughters and son thus we have 2 choices for front and 2 coices for back (as daughters need to separated) = 2*2=4
Case 2 Front seats occupied by a parent and son and back seats by daughters ,parent and son thus 2 possiblities for front seats and 2 for back Thus total =4
Case 3 front seats occupied by a daughter and a parent and back seats by son daughter and parent thus possibilites =2*2*3!=24
step 1. on the driver seat, there are 2 possibilities.
step 2: consider the rest of the seats. Categorize them.
Category 1: the other parent on front seat. back seat: daugthers have to separated. u have 2 choices. so cat1: 2
Category 2: son at front. back seat is similar to cat 1. 2 so cat 2: 2.
Category 3: one of the daughters is in the front seat. Remember either daughter could be in the front, so remember to multiply the back seat counts by 2 later.
Back seat: There are a parent, a son, and a daughter. They don't have any restrictions of seating, since a daughter is already seating in the front. so their arrangements is permutation of 3: i.e. 3!
So for Cat 3, there are totally 2*3! Multiply 3! by 2 is because the 2 daughters can exchange their seats.
Then total:
2*(2 + 2 + 2*3!) = 32.
fantastic! You make permutations and combinations sound super easy!