Mr. Cabigao planted 8/9 of his plot with vegetables. 1/4 of the vegetables planted were onions. To calculate the part of the plot planted with onions: Multiply the fractions 8/9 and 1/4 by multiplying the numerators and denominators, giving 8/36. Simplify by dividing the numerator and denominator by their greatest common factor of 4, giving 2/9. Therefore, 2/9 of the plot was planted with onions.