The answer is #85.00g#.

A solution's percent concentration is defined as the mass of solute divided by the total mass of the solution and multiplied by 100%.

#c = (mass_(solute))/(mass_(solution)) * 100%#, where

#mass_(solution) = mass_(solute) + mass_(water)#

Knowing that we have #100g# of solution, we can determine the mass of the solute by

#m_(solute) = (c * m_(solution))/100 = (15.00 * 100.0)/(100%) = 15.00g#

Therefore, the mass of water required for this solution is

#m_(water) = m_(solution) - m_(solute) = 100.0 - 15.00 = 85.00g#

This is a case where it is easy to eyeball the answer just because we are dealing with #100g# of solution...so a concentration of #20%# would have required 80g of water and 20g of #HNO_3#, a concentration of #8%# would have required 8g of #HNO_3# and #92g# of water, and so on...