Mr Hall sold 78 items at a car boot sale. 35% of his items did not sell,
How many items did Mr Hall have for sale?

120

A 35% decrease of (items he had for sale) is 78.
So 65% of (items he had for sale) is 78.

This means that 0.65 x ? = 78
We can work this out by doing 78/0.65 which is 120.

So I believe Mr Hall had 120 items for sale.

This is called reverse percentages if you want to watch some videos explaining it. Hope this helps!