Silly mistake

On a hard test, many errors are not about not knowing the maths — they are silly mistakes made in a hurry.
Knowing the common traps helps you catch them before they cost a mark.

The most frequent slip is answering the wrong quantity.
If you solve $2x = 10$ to get $x = 5$ but the question asks for $x + 1$, the answer is $6$, not $5$.
Always check what the question actually wants before choosing.

Sign and distribution errors are the next most common.
Dropping a minus turns $-(x - 5)$ into $-x - 5$ instead of the correct $-x + 5$.
Distribute across the whole bracket and keep track of every negative as you go.

So the fix is a quick habit: solve, then re-read and sanity-check.
Confirm the answer matches the quantity asked, the sign is right, and the size is reasonable.
A few seconds of checking saves marks that were never really in doubt.

Solve $3x = 12$, then give the value of $x + 4$.
(A) $4$
(B) $8$
Solving gives $x = 4$, but the question asks for $x + 4$, not $x$.
So the answer is $4 + 4 = 8$, which is (B); choosing (A) is the silly-mistake trap.

Simplify $-(x - 3) + 2x$.
(A) $x + 3$
(B) $3x - 3$
Distributing the minus gives $-x + 3$, so $-x + 3 + 2x = x + 3$.
Dropping the sign would wrongly give $3x - 3$, so the correct answer is (A).

A rectangle is $x$ cm wide and twice as long. If $x = 5$, what is its perimeter?
(A) $15$
(B) $30$
The length is $10$, so the perimeter is $2(5) + 2(10) = 30$; answering $15$ would come from forgetting to double or from finding just half.
So the correct answer is (B).