x 1= 100*80%=80
X2=60*80%=48 after the first pouring.
At this time, the concentration of brine is 48/ 100=48%.
The remaining x3 after the second pouring = 60 * 48% = 28.8.
At this time, the concentration of brine is 28.8/ 100=28.8%.
After the third pouring, the residual X4=60*28.8%= 10.368.
At this time, the concentration of brine is10.368/100 =10.368%.
Answer: The concentration of brine in the last cup is 10.368%.