Answer:
.0612 kg
Explanation:
I think your spec heat for lead should be  128 j /kg-K
Final lead temp is the same as the water 25 C Â Note that 1 K = 1 C degree
 so lead goes from 45 to 25 C   (or difference of 20 K)
Heat given up by the lead =
.5 kg * (20 K) Â * 128 j/ kg -C Â = 1 280 Â j
this heat is GAINED by the water going from 20 to 25 Â C
x kg * (5 C ) (4186 Â j/kg-C ) Â = 1 280 Â j Â
x = .0612 kg
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