Let
CP ------> cost price
SP ------> Selling price
we know that
SP= CP + 0.45CP
Mark up = 0.45CP
Ratio of Mark up to Selling price-------> 0.45CP/(CP + 0.45CP)
= 0.45/(1+0.45)-------> 0.45/1.45=0.3103
0.3103 multiplied by 100 = 31.03%
the answer is
31.03%