The error is that %d in the format specifier of both scanf and printf represents decimal, so it expects an int (in the case of scanf, a pointer to int)
Since you are declaring b and c as float, %d in the lines
scanf("%d",&b);
printf("%d lbs is %d kgs.",b,c);
should be changed to %f respectively.
A little piece of advice: use double in stead of float. It provides more precision, its performance is better in many machines. The downside is that it occupies more space than float, but that’s not a problem in most cases.
You should use %f in printf and %lf in scanf in case you are using double.
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solved decimal of numbers in c