**Solution:** Let for the both, rate of interest =Tk. x

So,

Interest on Tk. 100 in 1 year = x%

So, interest on Tk. 100 in 1 year = Tk. x

∴ ” ‘’ ‘’ 1 ‘’ 1 ‘’ = (x÷100)

∴ ” ‘’ ‘’ 400 ‘’ 3 ‘’ = (x×400×3)÷100

= Tk. 12x

Again,

Interest on Tk. 100 in 1 year = Tk. x

∴ ” ‘’ ‘’ 1 ‘’ 1 ‘’ = (x÷100)

∴ ” ‘’ ‘’ 400 ‘’ 3 ‘’ = (x×600×5)÷100

= Tk. 30x

According to the question,

12x + 30x =168

⇒ 42x = 168

⇒ x = 4

∴ The required rate of interest = 4%

Answer: 4%.