from the 8th to the 13th there are five numbers, and the difference between the 8th and the 13th is 20-12.5=7.5, so the difference between two neighboring number is 7.5÷5=1.5
from the 13th and the 25th there are 12 numbers, so the total difference is 12*1.5=18
so the 25th term is 20+18=38
