Здвох простих речень складіть одне складнопідрядне, де можливо-безсполучникове 1. he will solve the problem. he will work hard. 2. we will help our friends. we will finish our experiment. 3. substances decompose particularly in the presence of air and light. they must be kept in a dark place. 4. carbon dioxide is composed of the elements. the elements are carbon and oxygen.

If he works hard he will solve the problem.
Во втором вообще смысловой связи между предложениями не наблюдается. Достаточно просто поставить между ними "and".
Substances must be kept in a dark place because they decompose particularly in the presence of air and light.
Carbon dioxide is composed of the elements which are carbon and oxygen.