what strategy would be the best to solve this problem?

ann-marie bought apples. she kept one-third for herself and gave the rest to 4 friends. each friend got 2 apples. how many apples did ann-marie buy?

a. guess, check and revise
b. work backwards
c. use a formula
d. look for patterns

I’m not sure about that

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.