David and his wife each receives an 8 percent annual rise. If David receives a raise of Rs. 800 and his wife receives a raise of Rs. 840, what is the difference between their annual incomes after their raises ?

Let David's annual income be Rs. x and his wife's annual income be Rs. y
Then,
8% of x = 800
⇒ $$\frac{8}{100}$$x = 800
⇒ x = $$\left( {\frac{{800 \times 100}}{{8}}} \right)$$
⇒ x = 10000
And, 8% of y = 840
⇒ $$\frac{8}{100}$$y = 840
⇒ y = $$\left( {\frac{{840 \times 100}}{{8}}} \right)$$
⇒ y = 10500
∴ Required difference :
= [(10500 + 840) - (10000 + 800)]
= Rs. (11340 - 10800)
= Rs. 540