Given,
Two A.P's as
3,7,11,15,...
First term(a1)=3
Common difference(d1)=4
And,
1,6,11,16,...,
First term(a2)=1
Common difference (d2)=5
Now, common difference of the series of the common terms of the given A.P's is d=LCM(d1,d2)=LCM(4,5)=20
Now, series of common term is
11,31,51,71,...
So, 8th common term appearing in the both the series is
a8=11+(8−1)d
⇒a8=11+7×20=151