Answer
That day 80 boxes of standard strawberries and 55 boxes of deluxe strawberries were sold by the stand.
Work Step by Step
Let's assume that $x$ boxes of standard strawberries and $y$ boxes of deluxe strawberries had been sold that day. As $135$ boxes of strawberries was sold that day, Therefore-
$x+y$ = $135$ __eq.1
Now price for one box of standard strawberries is $ 7$ dollars and price for one box of deluxe strawberries is $ 10$ dollars, and total collection of the day was $1110$ dollars. Therefore-
$7 \times x +10 \times y$ = $1110$
i.e. $7 x +10 y$ = $1110$ __eq.2
Substituting for $x$ in eq.2 from eq.1-
$7 (135-y) +10 y$ = $1110$
i.e. $945 - 7y + 10 y$ = $1110$
i.e. $945+ 3 y$ = $1110$
i.e. $3 y$ = $1110-945$ = $165$
i.e. $y$ = $\frac{165}{3}$ = $55$
Substituting for $y$ in eq.1-
$x+55$ = $135$
i.e. $x$ = $135-55$ = $80$
Thus 80 boxes of standard strawberries and 55 boxes of deluxe strawberries were sold that day.
