Equivalents = Normality x Volume
Hence the neutralized equivalents => 40*0.10 => 4 mleq.
let M be the molarity of the acid then,
1 ml that means => mmoles of acid => 1*M
New Molarity would be => 1*M/250 => M/250
Now 25ml of this soln. => From the equal equivalents we get,
4 = (M/250)*25*2
we get M => 20