I was solving quant question from big book and was unclear with this answer of this problem. From the given data, there were two combination: (40+40+100) and (70+70+40). And only (70+70+40) could form a isosceles triangle and other couldn’t according to triangle property( 40+40 < 100). So, I choose option A(70+70>120) as there was only one possible triangle and question clearly states “sum of the measures of two angle of **triangle** RST that have equal measure” but the answer was D. And, so was the answer in Greg’s quant walk through.

I got logic why they choose D (40+40<120 and 70+70>140). So, My question/confusion is, Do we need to consider if the given combination of angle form a proper triangle while solving problem?