Solution:
Principal, P = 1000
Interest, I = 1,200 - 1,000 = 200
Time, T = 4 years

SI = PRT/100
⇒ R = (SI × 100)/(P × n)
⇒ R = (200 × 100)/(1000 × 4)
⇒ R = 20000/4000
⇒ R = 5

∴ Interest rate, R = 5%

∴ New Interest rate = 5% + 2% = 7%

New interest at 7% for 4 years:
New interest = (1,000 × 7 × 4)/100
= (28,000)/100
= 280 Tk.

New amount = Principal + Interest
= 1,000 + 280
= Tk. 1,280

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

Solution:
Principal, P = 1600 টাকা
Amount, A = 2304 টাকা
Time, n = 2 years
Rate, r = ?

We know,
A = P × (1 + r/100)n
⇒ 2304 = 1600 × (1 + r/100)2
⇒ (1 + r/100)2 = 2304/1600
⇒ (1 + r/100) = 48/40 [উভয়পাশে বর্গমূল করে]
⇒ 1 + r/100 = 6/5
⇒ r/100 = 6/5 - 1
⇒ r/100 = (6 - 5)/5
⇒ r/100 = 1/5
⇒ r = 100/5
∴ r = 20

∴ Interest rate 20%