# Tutor profile: Joseph H.

## Questions

### Subject: Chemistry

In what energy level is silicon's valence electrons?

3p.

### Subject: Calculus

A rock is dropped into the center of a circular pond, and the ripple moves outward at a rate of 9 m/s. What is the rate of change of area with respect to time when the ripple is 2m from the center?

This is a classic related rates problem. Essentially, we are looking for dA/dt, and we can find that by using the chain rule. dA/dt=dA/dr*dr/dt. We know what dA/dr is from the area of a circle, which is A=pi*r^2. The derivative of A with respect to r is just 2pi*r. We also know dr/dt, as it is given in the problem statement. Thus, the answer is just plugging in numbers, which is pi*3*2*2=12pi m^2/s

### Subject: Algebra

Mar was selling tickets so that her team may go to Florida for spring break. She sells ten more adult tickets than student tickets, and sold twice as many senior tickets as she did children's tickets. If x is the number of student tickets sold, write an expression for the number of adult tickets sold.

f(x)=10+x

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