Answer is 1/15 of the block.
Step by step. Adam used 3/5 so the portion left over is 1 whole or 5/5 less 3/5 = 2/5.
Now we will divide 2/5 by 6 pieces. When you divide fractions, you flip the divisor and multiply.
2/5 x 1/6 = 2/30 simplified to 1/15 of the block.
Problem solved!