Find the base of the following: 1. 20% of a number is 40
2. 40% of a number is 20
3. 70% of a number is 35
4. 80% of a number is 84
5. 20 is 5% of what number

help me please

I’m doing some practice exam questions for biology, and i’m not sure how to do these 3 parts to the question.

Ihave no idea how to solve these. plus there's another question that is 2) a 10 kilogram meteorite is collected in space and brought back to earth. it is composed of 1.25 kg of uranium-238 and 8.75 kg of lead-206. how many u-238 half lives have passed? what is the approximate age of the meteorite?

Dawn mistakenly took a multivitamin intended for men that contains 90 mg of vitamin c (or 100% of the vitamin c rda for men). if the multivitamin was dawn’s only source of vitamin c for the day, what percentage of the women’s vitamin c rda did she consume? a. 52% b.83% c.100% d.120%