Answer:
n = 10
Step-by-step explanation:
3n1!/(n-4)! = (n-1)!/( n-1-5)
3n(n-1)(n-2)(n-3)(n-4) / N-4 -------- case1 ( cancel ( n-4) from top and bottom)
= (n-1)(n-2)(n-3)(n-4)(n-5)(n-6) / n-6 --------- case 2 ( Cancel n-6 from top and
bottom and also cancel n-1,
n-2, n-3 with case 1)
3n = ( n-4)(n-5)
3n = n² - 5n - 4n + 20
3n = n² - 9n + 20
0 = n² - 9n - 3n + 20
0 = n² - 12n + 20
0 = (n-2)(n-10)
n = 2 ( not valid) n = 10
Therefore n = 10
