assumption method
step 1: find total if we assume all cakes are good
total cost (if all good) → 300 x $50 = $15 000
step 2: difference total and value
difference between total cost (if all good) and actual total → 15 000 - 13 000 = $2000
good cake → $50 spoiled cake → - $30
that means the difference (since 1 of the value is negative) will be and adding, not subtracting
difference in value → 50 + 30 = $80
step 3: divide to find cakes spoiled
difference total ÷ differenc value → 2000 ÷ 80 = 25
that means cake spoiled is 25