Question

calculate the volume of 4.0 M hydrochloric acid needed to make
100.0 mL of 0.10 M hydrochloric acid.

Answer 1

.100 L * .10 M/L = .010 M

.010 M / 4.0 M/L = ?

Answer 2

How many mols do you want? That's mols = M x L = 0.1 x 0.1 = ?
Then M HCl = mols/L. You know M = 4.0M and you know mols = ? from above, solve for L of the 4.0 M stuff. There is an easier way to do it. You should get the same answer eithr way.
mL1 x M1 = mL2 x M2
100 x 0.1 = mL2 x 4.0
mL2 = ?
(Note: Procedure 1 gives volume in L and procedure 2 gives volume in mL).