posted by JaneLee on .
1. If y varies directly as x, and y=30 when x=4, find the constant of proportionality.
2. If y varies inversely as x, and y=2 when x=10, find x when y=4.
3. If a varies jointly as b and c, and a=-10, when b=12 and c=2, find a when b=8 and c=1/2.