Solution:
Here,
Half-yearly interest rate = 5% ÷ 2 = (5/2)%

Now,
The first deposit of Tk. 1600 was made on 1st January.
It stays for 12 months, so it earns interest twice, once after 6 months, and again after 12 months.
So, it earns interest for 2 times (i.e., 2 half-years).

∴ A1 = P(1 + r/100)n
= 1600 × {1 + 5/(2 × 100)}2
= 1600 × {1 + (1/40)}2
= 1600 × (41/40) × (41/40)
= 1600 × 1681/1600
= 1681

Now,
The second deposit of Tk. 1600 was made on 1st July.
It stays for 6 months, so it earns interest only once (1 half-year).

∴ A2 = P(1 + r/100)n
= 1600 × (1 + 1/40)1
= 1600 × (41/40)
= 1640

Total amount = 1681 + 1640 = 3321
Total money deposited = 1600 + 1600 = 3200
∴Interest earned = 3321 - 3200 = Tk. 121

∴ The customer would have gained Tk. 121 by way of interest.

Solution:
Given,
Interest at 6% for 1 year:
Simple Interest, SI1 = Tk. 360
Interest rate, R = 6%
Time, T = 1 year (initial)

We know,
SI1 = PRT/100
⇒ 360 = (P × 6 × 1)/100
⇒ P = (360 × 100)/6
∴ P = Tk. 6000

So, the original sum (principal) is Tk. 6000.

Now,
Interest at 7% for 1 year:
SI2 = PRT/100
⇒ SI2 = (6000 × 7 × 1)/100
⇒ SI2 = Tk. 420

Additional interest = SI2 - SII
= 420 - 360
= Tk. 60