Question 1026197
a=1

d=2.5

t1=a =1

f2 = a+d =  3.5

t3 = a+2d = 4

t4 = a+3d = 5.5

t5 = a+4s = 7

t6 = a+5d = 8.5

t7 = a+6d = 10

tn = a+(n-1)d

t20 = 1+19*1.5

=28.5

Sum of n rems

sn = N/2[2a +(n-1)d]

S20 = 20/2 [2+19*1.5]

=305