Maths

7/10 of her income each month is on expenditure
5/21of her expenditure is spent on cosmetics. That is (7/10) x (5/21) = 1/6 of her income
1/6 of her expenditure is spent on transport. That is (7/10) x (1/6) = 7/60 of her income
The remainder of the expenditure, excluding cosmetic and transport = 1 - 5/21 – 1/6 =25/42
Food is (2/5) x (25/42) = 5/21 of her expenditure
Food is = (5/21) x (7/10) = 1/6 of her income
Cosmetic and food are 1/6 + 1/6 = 1/3 of her income
Her income $400 x 3 = $1200
She saves $1200 x 3/10 = $360

Miss Wu saves $360 each month

Comment:
How can she survive on $1200 per month in Hong Kong? She still manages to save $360! Please check the figures.
The break-down:
Income $1200
Expenditure $840
Saving $360

Cosmetic $200 (5/21 of expenditure)
Transport $140 (1/6 of expenditure)
Food = $200 ((5/21 of expenditure)
Other expenditure = $300