Build a marks dictionary and compute an average

Create chemMarks={}, then scan each score card. Store chemMarks[X.CardNo]=X.Chemistry. The lecture shows names as keys; the lab uses unique CardNo values so that repeated names would not overwrite another student. The dictionary keeps a computed view without altering the source cards.

chemMarks = {}
while (Table 1 has more rows) {
    Read the first row X in Table 1
    chemMarks[X.CardNo] = X.Chemistry
    Move X to Table 2
}
total = 0
count = 0
foreach k in keys(chemMarks) {
    total = total + chemMarks[k]
    count = count + 1
}
if (count > 0) {
    average = total / count
}

For the original 30 cards, Chemistry totals 2314 and the mean is 2314/30 ≈ 77.1333. Switching the lab field recomputes the values from those same cards. An empty input has no average; report it rather than dividing by zero.