Explanation:
20 pennies weigh 54 grams.
what is the weight of a single penny ?
well, 1/20 of the weight of the 20 pennies.
so,
54/20 = 27/10 = 2.7 grams
all his pennies weigh 918 grams.
how many pennies are there ?
as many as the times the weight of a single penny fit into the total weight.
so,
918 / 2.7 = 9180/27 = 340
he has 340 pennies.