What is the mass of an original 5.60-gram sample of iron-53 that remains unchanged after 25.53 minutes? 0.35 g 0.70 g 1.40 g 2.80 g?

0.70g

The half-life of iron-53 would be 8.51 minutes. So, in 25.53 minutes would be equal to: 25.53 min/ (8.51 minutes/ half-life)= 3 half-life.

Every half-life will reduce the original weight into half. So, the final weight would be:
final weight = original weight * 1/2 ^(time)
final weight = 5.6g * (1/2)^(3 half-life)
final weight = 5.6g * 1/8= 0.7g