There were a total of 840 guests at a wedding dinner.60% of them were adults while the rest were children.some boys left before the dessert was served.as a result,the percentage of adults increases to 70%.how many boys had left before the dessert was served?

Answer 1

70% of (840 minus boys)
... equals
... 60% of 840

588 - .7b = 504

84 = .7b