Question

in an A.P.,t7=20 and t18=42.then what is the common difference?

Answer 1

T7 is a+6d =20
T8 is a+7d =40
on substracting
-d = -20
d = 20

Answer 2

t7 = 20
t18 = 42
                             t7 = a + (n - 1) d
                            20 = a + 6d    .......................(1)
  and , 
                             t18 = a + (n - 1) d
                              42 = a + 17d      ..........................(2)

from (1) and (2) we get 
                                a = 8 and d = 2