Question

Tom has six banknotes.

Answer 1

Answer:

£5, £5, £10, £20, £20, £20.

Step-by-step explanation:

To make £20 with 3 notes he uses 2 5's and a 10 . That is the only way he can make £20 with 3 notes.

To make £55 with 4 notes he could use  2 £20's , £10 and a £5. That is the only way to make £55 with 4 notes.

So the possible list is:

20, 20 ,10 ,5, x, y    where x and y are the 2 extra notes.

He cannot make £25 with 3 notes so neither x nor y is £10.

He cannot make £25 with 4 notes so x and y cannot both be £5. But 1  must be £5 because he needs 2 £5 to make the £20.

So one must be £5 and the other must be £20.