There are $35$ girls and $15$ boys in a music club. $10$ new girls join the club. What is the percentage of girls in the club now?

Let's find the updated counts after new girls join the club.Initially:Number of girls = $35$ Number of boys = $15$ After $10$ new girls join:New number of girls = $35+10=45$ Number of boys remains the same = $15$ Total number of members in the club now = $45+15=60$ Percentage of girls now:
$$\text{Percentage of girls}=\left(\frac{\text{New number of girls}}{\text{Total members}}\right)\times 100$$
$$\text{Percentage of girls}=\frac{45}{60}\times 100=\frac{3}{4}\times 100=75\%$$