A cycloalkene (X) undergoes bromination with 1 mole of Br₂, consuming 2 moles of Br atoms (addition to the double bond).
The product (Y) has C:Br ratio of 3:1.
With 2 Br atoms present, the number of C atoms is 2 × 3 = 6.
Therefore, product Y has composition C₆H₁₂Br₂ with molar mass
=6(12)+12(1)+2(80)=72+12+160=244 g/mol.
Percentage of Br = (2 × 80 / 244) × 100 = (160 / 244) × 100
= 65.57%, which rounds to 66% (nearest integer).