## Elementary Technical Mathematics

$$25 .$$
We find $b$ by multiplying both sides by 2 as follows: $$A=\frac{1}{2}h(a+b)\Longrightarrow 2A=ha+hb \\\Longrightarrow hb=2A-ha.$$ By dividing both sides by $h$, we get $$b=\frac{2A-ha}{h}=\frac{2A}{h}-a.$$ By substituting, we have $$b=\frac{2*260}{13}-15=40-15=25 .$$