4th Grade Math
posted by Isabel .
At Mrs. Livingston's favorite grocery store, oranges are sold in bags of 10 and grapefruits are sold in bags of 6. Mrs. Livingston wants to buy an equal number of oranges and grapefruits.
What is the smallest number of each fruit Mrs. Livingston needs to buy in order to have an equal number of oranges and grapefruits?
a.12
b.20
c.30
d.60

The easy way is to take the answers and see if they are divisible by 10 and 6 to give a whole number (since the two fruits are sold by bags and not apiece).
12, not divisible by 10.
20, not divisible by 6.
30, divisible by 10 and 6.
60, divisible by 10 and 6.
To check ourself, 30 oranges can be bought in 30/10 = 3 bags and 30 grapefruit can be bought in 30/6 = 5 bags.
60 oranges can be bought in 60/10 = 6 bags while 60 grapefruit can be bought in 60/6=10 bags.
But the answer is 30 fruit since that is smaller than 60. 
I need to explain how i got theese prices.

Thank you SOOOO much!