Question

87. A student was asked to multiply a number by 12. But he multiplied by 21 and got his answer 72 more than the correct answer. The number to be multiplied was -
(a) 8
(b) 9
(c) 10
(d) 12

Answer 1

Answer:

• The number to be multiplied was 8.

Step-by-step explanation:

Let us assume:

• The number to be multiplied be x.
• The correct answer be y.

Given that:

• A student was asked to multiply a number by 12.

⟶ x × 12 = y

⟶ y = 12x ______(i)

• He multiplied by 21 and got his answer 72 more than the correct answer.

⟶ x × 21 = y + 72

⟶ 21x = y + 72

⟶ y = 21x - 72 ______(ii)

Finding the value of x:

Comparing eqⁿ(i) and eqⁿ(ii).

⇒ 12x = 21x - 72

⇒ 21x - 12x = 72

⇒ 9x = 72

⇒ x = 72/9

⇒ x = 8

∴ The value of x = 8

Answer 2

Answer:

Given :-

A student was asked to multiply a number by 12. But he multiplied by 21 and got his answer 72 more than the correct answer. The number to be multiplied was -

Solution :-

Let the wrong number be x

And correct number be y

x × 12 = y

12x = y

y = 12x

Then,

x × 21 = y + 72

21x = y + 72

y = 21x - 72

Comparing the both

12x = 21x - 72

12x - 21x = -72

-9x = -72

9x = 72

x = 72/9

x = 8