You can explain it more easily by taking an example.
Let there is 10 instruction(I1, I2, I3,.....I10) in the program to be executed and there is branch instruction at i3 to i8. Then calculate time take to execute this program.
Total number of cycle to execute i1 to i3 is (5+2-1)
Total number of cycle to execute i8 to i10 is (5+2)
Average time to execute = (13*50/10 = 6.5