The easiest and the best way to get to the answer would be to convert the denominators into either 100 or 10. This way the answers can be easily arrived at.
Mercury =87 24/25
= 87 96/100
= 87.96
Venus = 224 7/10
= 224.7
Mars = 686 49/50
= 686 98/100
= 686.98
I hope that the answers have come to your great help and the procedure is also clear enough for you to understand.