Do X/20 in an hour.

Wang Yi makes X/30 pieces per hour.

X÷(X/20+X/30)= 12 hours to complete.

Zhang made X/20 × 12=3X/5.

Wang made X/30 × 12=2X/5.

Zhang did 3X/5-2X/5=X/5 more than Wang.

Known by

X/5=60

X=300

This batch has 300 parts.