(1)假设小明全部做对,则小明做错的题目是:
(10×10-74)÷(10+3),
=26÷13,
=2(道),
则小明做对了:10-2=8(道);
(2)假设小华全部做对,则小华做错的题目是:
(10×10-22)÷(10+3),
=78÷13,
=6(道),
则小华做对了:10-6=4(道);
(3)假设小红全部做对,则小红做错的题目是:
(10×10-87)÷(10+3),
=13÷13,
=1(道),
则小红做对了:10-1=9(道);
所以他们一共做对了:8+4+9=21(道);
答:他们一共做对了21道题.