Question: The ratio of the present ages of a mother and son is 7 : 2. After 7 years, the ratio of their ages becomes 8 : 3. What will be the ratio of their ages after 11 years?

Solution:
Let,
The present age of the mother = 7x years
The present age of the son = 2x years

After 7 years, their ages will be:
Mother = (7x + 7) years
Son = (2x + 7) years

According to the question,
(7x + 7)/(2x + 7) = 8/3
⇒ 3(7x + 7) = 8(2x + 7)
⇒ 21x + 21 = 16x + 56
⇒ 21x - 16x = 56 - 21
⇒ 5x = 35
⇒ x = 35/5
∴ x = 7

Present age of mother = 7 × 7 = 49 years
Present age of son = 2 × 7 = 14 years

Now, after 11 years,
Mother's age = 49 + 11 = 60 years
Son's age = 14 + 11 = 25 years

∴ The ratio of their ages after 11 years,
= 60 : 25
= 12 : 5

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: The simple interest on a sum of money is Tk. 720 in 12 years. If the principal is doubled after 6 years, what will be the total interest at the end of 12 years?

Solution:
সরল সুদ সময়ের সাথে সমানভাবে অর্জিত হয়। যদি 12 বছরে 720 টাকা সুদ পাওয়া যায়, তাহলে 6 বছরে (অর্থাৎ অর্ধেক সময়ে) মোট সুদের অর্ধেক পাওয়া যাবে।

∴ প্রথম 6 বছরের সুদ = মোট সুদের অর্ধেক
= 720/2 = 360 টাকা

পরবর্তী 6 বছরে মূলধন দ্বিগুণ করা হলে, সুদও দ্বিগুণ হবে।
∴ পরবর্তী 6 বছরের সুদ = 360 × 2 = 720 টাকা

∴ মোট সুদ = প্রথম 6 বছরের সুদ + পরবর্তী 6 বছরের সুদ
= 360 + 720 = 1080 টাকা