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.