What percentage of strontium-90 remains in a sample after three half-lives have passed? 12.5% 15% 25% 60%?

Asap! how do you lengthen a pattern piece? (family and consumer science, sewing)

The scheme below is from a series of reactions that are part of a synthesis of vitamin a. answer the following questions with reference to this scheme. (i) what is "reagent a"? (ii) draw a step-by-step mechanism which explains the formation of compound c from compound b (iii) which reagents would you use to form compound e from compounds c and d (reagents b and c)? for each reagent suggested above in (ii) explain the role of the reagent in the reaction to (iv) form compound e. you may wish to do this by drawing a mechanism. 1. addition of reagent a но reagent a 2. н, о" thо oh нон-с compound a. compound b. compound c .ch-оh 1. reagent b "сно 2. reagent c сh oh compound e. compound d.

Which part of earth’s surface receives the most direct rays from the sun? a) equator b) ocean c) poles d) mountains