I used six lego bricks to pretend they were beads on an abacus. I started with six bricks on one side (the units) to make 6 then swapped it to the tens side to make 60.

I then took one brick and moved it over to the units side to make 51, then swapped them over (making 15).

I moved another from the 5 to the 1.

Then I repeated the process until I got to three on each side.

Answers: 6,60,15,51,42,24 and 33.