Question

An unknown number is doubled and added to 50, then subtracted by 20 to get 100. Which of the following is the unknown number?

• A. 30
• B. 35
• C. 40
• D. 45

Answer 1

The correct option is B 35

$\text{Let the unknown number be}x$.

Step 1: The number is doubled,
$\text{So, the new number will be 2}x$

Step 2: The number is added to 50
$\text{So, the new number will be 2}x\text{+ 50}$

Step 3: The number is subtracted by 20
$\text{So, the new number will be 2}x\text{+ 50 - 20}$

Step 4: Equate this expression to 100.
$2x+50-20=100$
$2x+30=100$


Step 5: Let's solve for $x$
Subtract 30 from both the sides
$2x+30-30=100-30$
$2x+0=70(\because 30-30=0;100-30=70)$
$2x=70$
$x=\frac{70}{2}(\because \text{bring 2 as a reciprocal to RHS}$
$x=35$
Hence, the unknown number is 35.