1. It takes Dustin 2 hours to shovel the snow from his driveway and sidewalk. It takes his sister 3 hours to shovel the same area. How long will it take them to shovel the walk if they work together?

Question

A: Working together, Dustin and his sister can shovel the walk in 1 1/5 hours, or 1 hour and 12 minutes?

2. A chemistry student needs to make a solution that is 70% water and 30% hydrochloric acid. The student's current mixture of 300 ml is 60% water and 40% hydrochloric acid. How much water must the student add to achieve his desired solution?

A: ?

A: ?

Answer 1

1. T = T1*T2/(T1+T2) = 2*3/(2+3)=1.2 h.

2. H = 0.4 * 300 = 120 mL in current
solution.
W = 0.6 * 300 = 180 mL of water in current solution.

W/(W+H) = 0.7
W/(W+120) = 0.7
W = 0.7W + 84
0.3W = 84
W = 280 mL of water in new solution.

280-180 = 100 mL of water added to new
solution.