Let the income of Mr. Manohar be Rs x.
Amount given to elder son = 20% of x
Amount given to younger son = 30% of x
Total amount given to both sons = (20 + 30)% of x = 50% of x
∴ Amount remaining with Mr. Manohar = (100 – 50)% of x
= 50% of x
= (50/100) x 100
= 0.5x
He gave 10% of the balance income as donation to a school.
Amount donated to school = 10% of 0.5x
= (10/100) × 0.5x
= 0.05x
∴ Amount remaining with Mr. Manohar after donating to school
= 0.5x – 0.05x = 0.45x
Mr. Manohar still had 1,80,000 for himself after donating to school.
∴ 180000 = 0.45x
∴
= 400000
∴ The income of Mr. Manoliar is Rs 4,00,000.