When a bakery opened this morning, the ratio of the number of pineapple muffins to mango muffins was 3: 2. after 144 pineapple muffins were sold, the ratio of the number of pineapple to mango muffins changed to 3: 5. how many pineapple muffins were there when the bakery opened this morning?

So
pinaple:mango=3:2

after 144 pinaple was sold (-144 pinaple)
the ratio changed to 3:5

one way is to convert the second number of both ratios to the same number, since they (mangos) didn't change
3:2=15:10
3:5=6:10

so
15 pinaple units-144=6 pinaple units
add 144 to both sides
15p=6p+144
subtract 6p (pinaple units) from bothsides
9p=144
divide both sides by 9
pinaple unit=16
there were 15 units in the begining so
in the beginning, ther were 15 times 16 pinaples or 240 pinaple muffins

240 is the answer