If the coefficients of three consecutive terms in the  expansion of (1+x)n  are in the ratio 1:7:42 then the value of n is

Let (r + 1)th, (r+2)th and (r + 3)th be three consecutiveterms.Then  nCr:nCr+1:nCr+2=1:7:42Now  nCr nCr+1=17⇒r+1n−r=17⇒n−8r=7 ........(i)And  nCr+1 nCr+2=742⇒r+2n−r−1=16⇒n−7r=13 ......(ii)On solving Eqs. (i) and (ii), we get n =55