In total, the numbers in this venn diagram would add up to 300. We first need to find the missing number for how many seniors said they were majoring in both math and science.
300 = 120 + x + 50 + 100
300 = x + 270
x = 30
Now that we know the missing number, we can find the probability that a high school senior is majoring in both math and science. The probability would be majoring in both divided by all high school seniors in the sample.
30 / 300 = 1 / 10
10%
Hope this helps! :)