Let the unknown number of plantains be x.
It is given that one plantain costs $0.49. It is required to find the number of plantains that can be bought at $7.84.
Since one plantain can be bought for $0.49, it follows that x plantains will be bought for:

Equate this expression to the given price:

Solve the equation for x:

Hence, the required number of plantains that can be bought for $7.84 is 16.
The correct answer is 16 plantains.