The random variable is $\displaystyle W = X_1 + X_2 + .... + X_{25}$ where $\displaystyle X_i$ ~ Normal $\displaystyle (\mu = 120, \, \sigma = 80)$.

You're expected to know that the sum of normal random variables is also a normal random variable:

Sum of normally distributed random variables - Wikipedia, the free encyclopedia.

You should therefore know that in this case, W ~ Normal $\displaystyle (\mu = 25 \times 120 = ...., \, \sigma = \sqrt{25 \times 80^2} = ....)$.

Use the distribution of W to calculate Pr(W > 2500).