Step-by-step explanation:
Amount of money spend in 1 day = x
So, amount of money spend in 7 days = (7x)
⇒ Amount of money spend in 1 week = (7x)
Amount of money saved in 1 week = y
∴ Income in 1 week = (7x + y)
So, income in 3 weeks = [3 x (7x + y)]
= (21x + 3y)
