To answer this question, you need to use the dilution equation, which is
C1 V1 = C2 V2
Rearrange to solve for V1
V1 = C2 V2 / C1
= 70% x 2.5 L / 100%
= 1.75 L round up to 1.8 L to the correct # of significant figures, which is 2.
Take 1.8 L of the 100% solution and add 0.7 L of water to make 2.5 L of the 70% solution.