# Problems of the Week June 10 to June 14

Jun 10, 2019 | Cambridge

Lower Elementary:

Question: The Moon is 3500 kilometres wide. The Earth is 12 700 kilometres wide. How many kilometres wider is the Earth than the Moon?

Solution:  To solve the problem, we want to set up a vertical equation, start at the ones place, and move left. We know that 0 – 0 = 0, so we can fill in the ones and tens places and move on to the hundreds. 7 hundred minus 5 hundred is 2 hundred, and 12 thousand minus 3 thousand is 9 thousand. Altogether, that makes 9200 km.

Upper Elementary:

Question: For every 2 revolutions Pluto makes around the Sun, Neptune makes 3. How many revolutions does Pluto complete if Neptune completes 21?

Solution:  The ratio of Pluto’s revolutions to Neptune’s revolutions is 2:3. To find out how many revolutions Pluto completes, we first need to find out how many times Neptune makes 3 trips. 21 ÷ 3 = 7, so Neptune make 7 sets of 3 revolutions. That means that Pluto makes 7 sets of 2 revolutions, which means that Pluto revolves around the sun 7 × 2 = 14 times.

Middle School:

Question: Callisto, Europa, Ganymede, and Io are moons of Jupiter. Ganymede is not the farthest moon from Jupiter. Callisto’s orbit is not the closest orbit to Io’s. Europa is closer to Jupiter than Ganymede is. Io is the closest to Jupiter. What is Jupiter’s farthest moon?