EXPLANATION :
The banana costs $0.50 and the mango costs $2.
Let B = the number of bananas
M = the number of mangos
She bought twice as many bananas as mangos.
So B = 2M
The total cost of bananas and mangoes is $12
Then :

Since M = 4, B will be :
B = 2M
B = 2(4) = 8
ANSWER :
The number of bananas is 8 and the number of mangoes is 4.