We are given a problem that can be solved by a system of equations. Let N be the number of nickels, D the number of dimes, and Q the number of Quarters. Since in total he has 4.2, this means mathematically:

We are told that he has 6 more dimes than nickels, this can be written like this:

We are told that he has three-time Quarters than nickles, this is:

Now, if we replace equation (2) and (3) in equation (1), we get:

Solving for N, we get;

Replacing the value of N in equation (2), we get:

Now we replace the value of N in equation (3):

Therefore, he has, 0.42 in nickels, 2.52 in dimes, and 1.26 in quarters.