Write down the balanced reaction first
3NaOH + H3PO4 makes Na3PO4 + 3H2O
Given 0.10M NaOH (0.10mol/lit NaOH)
0.2M H3PO4 ( 0.2 mol/lit H3PO4)
15 ml of H3PO4 or 0.015 lit of H3PO4
Find volume of NaOH needed to neutralize the solution
Relate moles of H3PO4 with the moles of NaOH with the helps of coefficients in front of NaOH and H3PO4 seen in balanced equations.
moles ↔ Litre use definition of molarity (moles/litrs)
Start with what is given for H3PO4
0.015 lit of H3PO4 x 0.2 moles of H3PO4 / lit of H3PO4 x 3 moles of NaOH/1 mole of H3PO4 x lit of NaOH/0.1 mol of NaOH = 0.09 lit or 90 ml.
The same colors in above equations cancel our leaving lit of NaOH as final answer.