1. If the current age is x, then n times the age is nx.
2. If the current age is x, then age n years later/hence = x + n.
3. If the current age is x, then age n years ago = x  n.
4. The ages in a ratio a : b will be ax and bx.
5. If the current age is x, then 1/nof the age is x/n.
1. 

Let Rohan's present age be X years. Then,
Rohan's age after 15 years = (X + 15) years
Rohan's age 5 years back = (X  5) years
∴ X + 15 = 5(X  5)
⇒ X + 15 = 5X  25
⇒ X = 10.
Hence, Rohan's present age = 10 years.
2. 

Let the age of the younger person be X years.
Then, age of the elder person = (X+16) years.
∴ 3(X  6) =(X + 16  6)
⇒ 3X – 18 = X + 10
2X = 28
X = 14
Hence, their present ages are 14 years and 30 years.
3. 

Let Ankit's age be X years. Then Nitin's age = 240/X years.
∴ 2 x 240/X X = 4
⇒ 480  X² = 4X
X² + 4X 480 = 0
⇒(X + 24)(X  20) = 0
⇒ X = 20
Hence, Nitin age =(240/20) years = 12 years.
4. 

Let the son's present age be X years. Then, father's present age = (3X +3)years
∴(3X+3+3) = 2(X+3)+10
3X + 6 = 2X + 16
X= 10
Hence, Father's present age = (3X+3) = (3 x10 + 3) = 33 years
5. 

Let son's age after 8 years ago be X years. Then, Sohit's are 8 years ago = 4X years.
Son's age after 8 years =(X+8)+8 =(X+16) years.
Sohit's age after 8 years =(4X+8)+8 =(4X+16)years.
∴ 2(X+16) = 4X+16
X = 8 years.
Hence, son's present age =(X+8) = 8+8 = 16
Sohit's present age = 4X+8 = 32+8 = 40 years.
6. 

Let Gaurav's and Sachin's age one years be 6X and 7X years respectively. Then,
Gaurav's age 4 years hence = (6X+1) + 4 = (6X + 5) years
Sachin's age 4 years hence = (7X+1) + 4 = (7X + 5) years
∴ (6X + 5)/(7X + 5) = 7/8
⇒ 8(6X + 5) = 7(7X + 5)
⇒ 48X + 40 = 49X + 35
⇒ X = 5
Hence, Sachin's present age = 7X+1 = 36 years
7. 

Let the ages of Abhay and his father 10 years ago be X and 5X years respectively. Then,
Abhay's age after 6 years = (X+10) + 6 =(X+16) years
Father's age after 6 years = (5X+10) + 6 = 5X+16 years
∴(X+16) = 3/7(5X+16)
⇒ 7(X+16) = 3(5X+16)
⇒ 7X + 112 = 15X + 48
⇒ 8X = 64
⇒ X = 8
Hence, Abhay's father's present age =(5X+10) = 50 years
8. 

Let the ages of Jayant, Prem and Saransh 10 year ago be 2X,3X and 4X years respectively.
Then,(2X+10) + (3X+10) + (4X+10) = 93
⇒ 9X = 63
⇒ X = 7
∴ Saransh's present age =(4X+10) = 38 years
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