solution:
if 100 gram mushroom contains 1.2 gram carbs
then,
1 gram mushroom contains 1.2/100=0.012 gram of carbs
then,
25 gram mushroom contains
25*0.012 gram of carbs=0.3
to check,
100/4=25
25 grams contains 0.3 gram carbs
100 gram contains 4*0.3=1.2 gram carbs(which is given)