Karen spent half her weekly allowance, a, to buy a book. She then earned $9 walking her neighbor's dog for the weekend. She ended up with $13. Determine Karen's weekly allowance, using the equation: 1/2a+9=13. D: a=4
C: a=8
B: a=11
A: a=44

Im taking a test and i need help!

Planet earth has a mass of 5.9726x10²⁴ kg.then: 5.9726x10²⁴ kg=100% of the total earth mass.if the water´s mass=5x10-4 % of the total mass, we can solve this problem by the rule of three.5.9726x10²⁴ %*10⁻⁴ %x=(5.9726x10²³ kg)(5*10⁻⁴  %) / 100 %=2.9863x10¹⁹ kganswer:   the mass of the planet´s water would be   2.9863 x 10¹⁹ kg.