a3=a1+2d, a8=a1+7d,
so: a3+a8=2a1+9d=24
cause:Sn=a1*n+n(n-1)d/2
then: s10=a1*10+10(10-1)d/2
=10a1+45d=5(2a1+9d)=5*24=120
the answer is 120