At 7 p.m. the temperature was 4C°. By 8 p.m.it had increased 20°, by 9p.m. It has decreased y
1 Cº, by 10pm it has decreased by 15 Cº. What
was the temperature at 10p.m.?

8 C°.

Step-by-step explanation:

7 p.m. was 4 C°. 1 hour later, it had increased 20 C°, so we take 4 added by 20. That gives us 24 C°. 1 hour later, it has decreased by 1 C°. So we take 24 minus 1. That gives us 23°C. And then by 1 hour later, it has decreased by 15°C. So we take 23 minus 15, equals to 8°C.