Educational Round 10 (Goodbye cstuart and kai824) (0/700)

hi cbr, the first edu round of 2022, edu round 10 (goodbye cstuart and kai824) will start in 2 jan, 12.30 am and end in 12 jan 11.59 pm. there will be 6 problems (1 of them is split into 2 parts). setters are cstuart, kai824, errorgorn, oolimry(?) and mr li(?). problems are prepared by cstuart with help from kai824 and errorgorn for the problems they set. editorials will be published so you may use this to mug for noi sel. problems may not be arranged in increasing order of difficulty. glhf