How do you do multiplication ad division expressions

Serena is making an experiment. for that, she needs 20 grams of a 52% solution of salt. she has two large bottles of salt water: one with 40% and the other with 70% of salt in them. how much of each must she use to make the solution she needs? i am in need of

Solve for x in the equation x2 - 12x + 36 = 90. x= 6+3x/10 x=6+2/7 x= 12+3/22 x = 12+3/10

The half-life of a certain material is 0.004 second. approximately how much of a 20-gram sample will be left after 0.016 second? a. 5 grams b. 16 grams c. 0.32 gram d. 1.25 grams