As you see in the image i want to know how to do such questions fast as i dont get much time in the exam. Please help me sir

I dont think there is any definite fast way to do such problems, I for myself have never used a trick for such problems

But you can reduce errors if you do it systematically
there are two ways
--- start from the biggest triangle and move inwards counting
--- start from the smallest triangle and move outwards counting

One way to do it faster is to exploit the symmetry of the figure to count the smaller triangle, but it may result in overcounting of bigger triangles so be careful