18. The minimum charge of telephone per month up to 100 calls is Rs 180 then the
cost is Rs 3 per call of 4 minutes. A family paid Rs 1,188 as telephone charge
including 10% VAT. Find the total number of calls made.​

geometry: triangles

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call the larger triangle abc

then the smaller triangle cde is similar to abc (all angles are equal, aaa).  

ab is the side that is 36, and we wish to find de, the side corresponding to ab

area_cde = area_abc - area_cde --> area_abc / area_cde = 2

let x=length of de

ratio of areas = (ratio of sides)^2

+2+=+%2836%2fx%29%5e2+

+2+=+1296%2fx%5e2+

++x%5e2+=+1296%2f2+=+648+

25.45584

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