Charlie, Mo and Andrzej share a flat.Charlie pays 25% of the rent.Mo pays 1/2 of the rent.Andrzej pays £450.How much do they pay altogether for the rent?

Altogether, they pay $1800 for the rent.

The first two people pay a combined total of 75% for the rent. That leaves Andrzej with 25% to pay and he paid 450.
So 450 is 25% or 1/4 of the rent.

450 x 4 = 1800 for the total rent