Final answer:
In Sarah's science experiment, to maintain the 5:3 ratio given in the instructions, she should use 25 milliliters of "substance A" when she has 15 milliliters of "substance B".
Step-by-step explanation:
Sarah is conducting a science experiment and needs to figure out how many milliliters of "substance A" she should use when the directions tell her to mix 5 parts of "substance A" with every 3 parts of "substance B". If she has 15 milliliters of "substance B", then to maintain the correct ratio, she will need to calculate the corresponding amount of "substance A". The ratio given is 5:3, which means for every 5 milliliters of "substance A", there should be 3 milliliters of "substance B". To find the equivalent amount when 'substance B' is 15 milliliters, we set up a proportion:
5 milliliters of A / 3 milliliters of B = x milliliters of A / 15 milliliters of B
To solve for x, we cross-multiply:
5/3 = x/15
3x = 75
x = 75 / 3
x = 25 milliliters
Therefore, Sarah should use 25 milliliters of "substance A".