total at first → 200 files
"40% of them were blue " blue at first → 200 x 40% = 80
"and the rest were yellow." yellow at first → 200 - 80 = 120
"The shop owner bought 100 more blue files. "
blue in the end → 100 + 80 = 180
"What percentage of the files were blue in the end?"
total in the end → 200 + 100 = 300
blue in the end % → blue in the end / total in the end x 100% = 180 / 300 x 100% = 60%