Jack knows that he has 40 coins in his pocket all either nickels or dimes If the total amount of money he has equals 3.50 how many nickels does he have?

Let x be the number of nickles. So, the number of dimes is 40 - x. So,

0.05x + 0.10(40 - x) = 3.50

0.05x + 4 - 0.10x = 3.50

- 0.05x + 4 = 3.50 subtract 4 to both sides;

-0.05x = - 0.50 divide by - 0.05 to both sides;

x = 10

Thus, Jack has 10 nickles and 30 dimes.