Find the error and explain why its wrong

The last part.

Step-by-step explanation:

The error is only at the last part. Basically the answer part.

The answer should be —

We don't multiply the square roots when adding or subtracting. That'd contradict the rules.

Here, we let the square root of five as x-term.

As we add up, we would get 13x. Thrn we convert x-term back to square root of five.

The answer would be —

Remember that we don't multiply the square roots when adding or subtracting.

(a) the volume of the cone is 20/3 in3

hence, option a is correct.