First figure out how many ounces of fruit fit in three bags by multiplying the number of bags by the number of ounces per bag: 49 oz/bag * 3 bags = <<49*3=147>>147 oz
Since Marta wants the same number of apples and oranges in each bag, we know the fruits always come in pairs. Find the weight of each pair of fruit by adding the weight of one apple to the weight of one orange: 3 oz + 4 oz = <<3+4=7>>7oz
Now figure out how many pairs of fruit Marta needs to fill the bags by dividing the total weight in the bags by the weight of each pair of fruit: 147 oz / 7 oz = <<147/7=21>>21
Now multiply the number of apples Marta needs to buy (21) by the weight per apple (4 oz) to find how many ounces she should buy: 21 * 4 oz = <<21*4=84>>84 oz
#### 84