Question

solve: $2(2x + 9) = 5x + 72$
step 1: $4x + 18 = 5x + 72$
step 2: $9x + 18 = 72$
step 3: $9x = 54$
step 4: $x = 6$
which is the first incorrect step in the solution shown above?
a. step 1
b. step 2
c. step 3
d. step 4

Explanation:

Step1: Check Step 1

Left side expansion: $2(2x + 9) = 4x + 18$, correct.

Step2: Check Step 2

Should subtract $5x$ from both sides: $4x - 5x + 18 = 72 \Rightarrow -x + 18 = 72$, not $9x + 18 = 72$. Step 2 is wrong.

Step3: Confirm first incorrect step

Step 2 is the first mistake.

Answer:

B. Step 2