A Stock solution has a total volume of 1.5l. It is diluted by removing 10 ml of the stock and adding 90 ml of water. the diluted solution has a concentration of the solute of 40mg/l

a, what is the concentration of the solute in the stock solution?
b, How much solute is present in the total 1.5l of stock?

Technically you don't know because the solution volumes are not additive; i.e., 10 mL + 90 mL is not 100 mL. However, the author of the problem meant this to be a 10 to 100 dilution which is a dilution of 10x so the 40 mg/L must be 10 x stronger or 400 mg/L.

b. 400 mL/L x 1.5 L = ?
And by the way that is 1.5 L and not 1.5l.