Probability of at-least 2 girls = Probability of 2 girls + Probability of 3 girls + Probability of 4 girls + Probability of 5 girls = 1 - (Probability of no girl + probability of 1 girl)
We know:
probability when no girl = 5C0 * (1/2)^0 *(1/2)^5 = 0.03125
probability when one girl = 5C1 * (1/2)^1 *(1/2)^4 = 0.15625
probability when at least 2 girls = 1-(0.03125+0.15625)
= 0.8125(approx)