Ozonolysis of an alkene cleaves the carbon-carbon double bond, converting the double-bonded carbons into carbonyl carbons. To find the original alkene, we remove the oxygen atoms from the carbonyl groups of the ozonolysis products (propanal and pentan-3-one) and join the two fragments with a double bond. Propanal ($\text{CH}_3\text{CH}_2\text{CHO}$) provides the $\text{CH}_3\text{CH}_2\text{CH}=$ fragment, and pentan-3-one ($\text{CH}_3\text{CH}_2\text{COCH}_2\text{CH}_3$) provides the $=\text{C}(\text{CH}_2\text{CH}_3)_2$ fragment. Joining these gives the alkene $\text{CH}_3\text{CH}_2\text{CH}=\text{C}(\text{CH}_2\text{CH}_3)_2$.