The sum of the first 20 terms common between the series 3+7+11+15+... and 1+6+11+16+... is

3, 7, 11, 15, 19, 23, 27, 31... difference=41, 6, 11, 16, 21, 26, 31 difference=5common terms are 11,311stcommon term, a=11Common difference, d=20Sn=n2(2a+(n𕒵)d)so for sum of first 20 termsS20=202(2×11+(19×20)=10×(22+380)=10×402=4020