Use the key to open the box next to the door. Take the toilet paper.
Select the toilet paper, then tap it to remove the paper.
Look at the scale. You want to make the scale reach the red arrow of 500g.
Put the empty vase and the toilet paper roll (without toilet paper) on the scale.
Open the door.
Proceed to the next 100 Toilets level!
Hit like if the 100 Toilets Level 42 walkthrough helped you! Thanks :)