You need the ratios of products and reactants, but in this case it looks like 1 mole + 1 mole -> 1 mole + 1 mole.
In that case, you can see that you have used up 2.5 moles of methanoic acid - this will have reacted with 2.5 moles of ethanol to give 2.5 moles of each product. Therefore, as 2.5 moles of ethanol are used up, there will be 6.25-2.5 moles in the mixture - i.e. 3.75 moles.