$20r+60t$

$4\left(x+3\right)-2\left(x+3\right)=12$

$\frac{3x}{2}-7=\frac{x}{4}-2$

$-1\cdot\left(-3\right)^4:\left(-27\right)$

$\lim_{x\to\infty}\left(\frac{1}{1+ae^{-cx}}\right)$

$i\:\int_0^{\frac{\pi}{8}}sec^2\:\left(x\right)\:dx$

$\lim_{x\to0}\left(\frac{7^x-6^x-1}{x^2-1}\right)$