We don't know the total number of trees, so we will assign that quantity the variable x. Now, let's find the LCD of 2/5 and 1/4. It is 20.
2/5 = 8/20
1/4 = 5/20
The number of Gala apple trees is 2/5 x = 8/20 x
The number of Ambrosia apple trees is 1/4 x = 5/20 x
The total number apple trees, which we call x, is the sum of the number of Gala, Ambrosia, and Granny Smith apple trees.
x = 8/20 x + 5/20 x + 350
x = 13/20 x + 350
Can you now solve this linear equation for x?