How much money becomes Tk 1280 as profit-principal in 12 yr at 5% simple per annum?

Simple interest over 12 years at 5% is $0.05 \times 12 = 0.6$ of the principal, so amount $= P + 0.6P = 1.6P$. Setting $1.6P = 1280$ gives $P = \text{Tk } 800$.