Suppose the monthly instalment = Rs x
Time=312years=72×12months=42 months
Total amount deposited = Rs 42×x=Rs 42 x
Principal for 1 month = Monthly instalment ×n(n+1)2
=Rsx×42(42+1)3
=Rs 21x×43
= Rs 903 x
Interest=P×R×T100
=Rs903x×19×1100×12×2=Rs5719x800
According to question,
Amount received on maturity = 78638
⇒ Total money deposited + Interest = 78638
⇒42x+5719800x=78638
⇒33600x+5719x800=78638
⇒39319x=78638×800
⇒x=78638×80039319
⇒x=Rs 1600
∴ Monthly istalment = Rs 1600