First, we can eliminate the percent of students who brought both: 100-10=90

Then, we can solve for the percentage of the students bringing an umbrella only

We can see that number of umbrellas compared to raincoats is 2:1

So, we can turn the amount of raincoats into a fraction then solve the problem!

The fraction is: 2/3 because we do 2+1=3 for how we are going to divide the rest 90% into

Finally, we do 90x2/3=60% for the final answer!