Question: If 2/3 of X = 75% of Y = 0.4 of Z, then find the ratio X : Y : Z.

Solution:
According to the question,
(2/3) × X = 75% of Y
⇒ 2X/3 = 75Y/100
⇒ 2X/3 = 3Y/4
⇒ X/Y = 9/8

Now,
75% of Y = 0.4 of Z
⇒(75Y/100) = (2Z/5)
⇒ (3Y/4) = (2Z/5)
⇒ 15Y = 8Z
⇒ Y/Z = 8/15

X : Y : Z = (9/8) : 1 : (15/8)
Multiply all terms by 8:
X = 9 × 1 = 9
Y = 1 × 8 = 8
Z = (15/8) × 8 = 15

Therefore,
X : Y : Z = 9 : 8 : 15

Question: A bank offers 5% compound interest calculated half-yearly. A customer deposits Tk. 1600 on 1st January and another Tk. 1600 on 1st July of the same year. How much interest will he earn at the end of the year?

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.

Question: Find the simple interest on BDT 15,000 at 8% per annum for 5 months.

Solution:
Principal, P = 15,000 Taka
Time, n = 5 months = 5/12 years
Rate of interest, r = 8% = 8/100

Simple Interest, I = P × n × r
= 15,000 × (5/12) × (8/100)
= (15,000 × 5 × 8)/(12 × 100)
= 600,000/1200
= 500

∴ The simple interest is Tk. 500.