The number of cakes Mrs Darby can decorate in a 30-hour week is 22.5 cakes.
Time conversion
1 hour = 60 minutes
1 1/3 hours = 60 × 1 1/3
= 60 × 4/3
= 240/3
= 80 minutes
30 hours = 30 × 60
= 1800 minutes
Number of cakes decorated = 1800 / 80
= 22.5 cakes
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