15

Explanation

Since 45% + 30% = 75%, 75% of the children were first and second graders. The percentage of third graders = 100% – 75% = 25%. \(\text { The number of third graders }= \frac{25}{100} \times 300=75\). \(\text { The number of first graders }=\frac{30}{100} \times 300=90\). So there were 90 – 75 = 15 more first graders than third graders.

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