Grid based movement in Snake Game C++ - c++

Question

I have basic knowledge of c++ and am trying to create a simple Snake game that will run on a 84 x 48 pixel LCD screen. The game works fine, but there is one thing I have been stuck on for a while, and I haven't found any help in any previous post:
so far, the movement of the snake is free, i.e. it moves pixel by pixel, meaning that, the snake being composed of 3 x 3 pixel squares, most of the times it doesn't automatically align with the fruit. I would like to implement a "grid system", so turn my 84 x 48 game space into a 28 x 16 grid in which the snake moves and eats the fruit without the player having to adjust its trajectory many times before getting the right alignment with the fruit.
Here's the part of my code in which I print the snake to the screen:
void Snake::head_init(int size)
{
_size = size;
_x = (GRIDX/2 - _size/2)*_size;
_y = (GRIDY/2 - _size/2)*_size;
printf("Snake initially at x = %i, y = %i\n", _x,_y);
}
void Snake::draw(N5110 &lcd)
{
lcd.drawRect(_x,_y,_size,_size, FILL_BLACK); // draw head
for(int k = 0; k < _tail_length; k++) {
lcd.drawRect(_tail_x[k],_tail_y[k],_size,_size, FILL_BLACK); // print tail
}
}
GRIDX = 28, GRIDY = 16, _size = 3
I tried to multiply _x and _y by the size of the snake when printing it, but this doesn't give me the result I am aiming to.
Also, I would like the fruit to spawn in the same 28 x 16 grid.
Here is how my fruit is printed for now:
void Fruit::init(int size)
{
_size = size;
srand(time(NULL)); // initialise random number generator for fruit positions
_x = rand() % 80 + 1; // generate a random x coordinate for fruit within the frame x range
_y = rand() % 43 + 1; // generate a random y coordinate for fruit within the frame y range
printf("Fruit initially at x = %i, y = %i \n", _x, _y); // print the random fruit coordinates over terminal
}
void Fruit::draw(N5110 &lcd)
{
lcd.drawSprite(_x,_y,3,3,(int *)fruit); // draw fruit at random position
}
Any help would be very appreciated