Solution :
Total number of integral points inside the square $$OABC\,\, = 40 \times 40 = 1600$$
Number of integral points on $$AC$$

$$=$$ Number of integral points on $$OB$$
$$=40$$ [namely (1, 1), (2, 2) ... (40, 40)]
Number of integral points inside the $$\Delta OAC$$
$$ = \frac{{1600 - 40}}{2} = 780$$