Final answer:
Yumi can plant 320 apple trees if she uses all her coins.
Step-by-step explanation:
To find out how many apple trees Yumi can plant, we need to divide the total number of coins she has, which is 6,720, by the cost of planting an apple tree, which is 21 coins. The division equation that represents this problem is 6,720 ÷ 21 = X, where X represents the number of apple trees Yumi can plant. Let's perform the division:
Divide 6,720 by 21:
6,720 ÷ 21 = 320
Therefore, Yumi can plant 320 apple trees if she uses all her coins.