college discrete mathematics problem

Question

One quarter of the five-element subsets of {1, 2, 3, . . . , n}

contain the element 7. Determine n (≥ 5).

I have setup the equation as

1/4*(n,5)=(n-1,4)

but do not know how to solve for n

Answer 1

 

Given a relation, R on A ={a,b,c} as the following. Determine whether the

following relations is reflexive, symmetric or transitive.

a) {(a,b),(a,c),(b,c),(c,a)}

b) {(a,a), (b,b),(b,c),(c,c)}

c) {(a,a),(c,c),(b,b)}

d) {(a,a),(b,b), (b,c),(c,c),(c,b)}

e) {(b,b),(b,a),(c,a),(b,c),(a,a), (a,c)}