Alternate method :
difference of first series = 4, difference of second series =5
so difference of common series will be always LCM (4,5 )= 20 .
by seeing both series ...we can easly find first term = 21,
and last term of common series will be less than or equal to 417 ,bcoz common series will go up to common of both and first sequence is till 417 .
series like 21 ,41,61.... 401....(bcoz 401%20=1 ,)
so 401= 21+ (n-1)20
380/20= (n-1)
n= 20 hence answe is 3)