This was a tricky problem, but many of you tackled it successfully. Your solutions depended on whether you assumed that you could cut the paper or not. Rachel from Charter Primary said:

I solved this problem by drawing a net.

The length of my cuboid was $38 \; \text{cm}$. The width of my cuboid was 14cm. The depth of my cuboid was $1\; \text{cm}$.

I tried to make the unused space as small as possible.

So, Rachel's solution kept the paper in one piece (as you would usually do when you wrap a present, for example).

This gives a cuboid with the maximum possible surface area ($1168 \;\text{cm}^2$).I wonder how you arrived at this solution, Rachel? How many differently-sized cuboids did you try before you discovered this was the largest? Rohaan from Longbay Primary looked at it in a different way, assuming that you could cut the paper:

We decided to find a cuboid that that had a surface area that
matched with the wrapping paper ($1200 \; \text{cm}^2$).

We made a starting estimate of a cuboid that was
$1\times20\times20$. It had a surface area of $880 \;
\text{cm}^2$.

Then we thought there could be a bigger cuboid that would fit.
We wondered by how much the cuboid's surface area would go up if we
changed its measurements from $1\times20\times20$ to
$2\times20\times20$. It went up $80 \; \text{cm}^2$.

We thought if we changed it to $3\times20\times20$ it would go
up by $80 \; \text{cm}^2$ again. It did. We went up until we
reached $5\times20\times20$ which had a surface area of exactly
$1200 \; \text{cm}^2$. It matched the surface area of the wrapping
paper.

Just to make sure it fitted, we drew up the surface area
($1200 \; \text{cm}^2$) on a piece of A3 paper. It fitted!

Did anyone try to find the cuboid with the largest volume that could be wrapped up in this paper? That's another challenge for you!