The output is displayed as two images (not three) wherever the operating system decides to put them. This is because you call imshow() twice. Anyway you're doing it wrong (i.e. slow and not vectorized). Plus you probably don't need it to be 255 - you probably want it to be binary (logical) which will make it so much easier to process those pixels. If you make it 255 you'll just have to convert to 0 and 1 sooner or later anyway. Try my code:
binaryImage = (I > 50) & (I < 70);
count = sum(binaryImage(:));
figure;
subplot(1,2,1);
imshow(I);
title('Original Image', 'FontSize', 24);
subplot(1,2,2);
imshow(binaryImage);
title('Binary Image', 'FontSize', 24);
% Enlarge figure to full screen.
set(gcf, 'Units', 'Normalized', 'OuterPosition', [0 0 1 1]);
