  We introduce an $L_q(L_p)$-theory for the quasi-linear fractional equations
of the type $$ \partial^{\alpha}_t u(t,x)=a^{ij}(t,x)u_{x^i x^j}(t,x)+f(t,x,u),
\quad t>0, \,x\in \mathbf{R}^d. $$ Here, $\alpha\in (0,2)$, $p,q>1$, and
$\partial^{\alpha}_t$ is the Caupto fractional derivative of order $\alpha$.
Uniqueness, existence, and $L_q(L_p)$-estimates of solutions are obtained. The
leading coefficients $a^{ij}(t,x)$ are assumed to be piecewise continuous in
$t$ and uniformly continuous in $x$. In particular $a^{ij}(t,x)$ are allowed to
be discontinuous with respect to the time variable. Our approach is based on
classical tools in PDE theories such as the Marcinkiewicz interpolation
theorem, the Calderon-Zygmund theorem, and perturbation arguments.
