MATLAB sum of pixel between 0 and 100 intensity

Hello. I trying to generate an image into histogram and in that image i have to calculate the sum of pixels. Then after that i have to change it to grayscale but how do i calculate sum of pixel between 0 and 100 intensity of the grayscale image? I tried adding the code y=(x<=100) but the value suddenly change to binary.

ths is my code

clc;

clear;

I=imread('image.jpg');

imhist(I);

pixelCounts=imhist(I);

numberOfPixels = sum(pixelCounts)

x=rgb2gray(I);

y=(x<=100)

Answer 1

[KSSV]

If I is your image.

idx = I >= 0 & I <= 100 ;  % this gives logical indices of the pixels present 
N1 = nnz(idx(:)) ;     % this gives total 
N2 = sum(idx(:)) ;     % this gives total