No wonder you didn't do well in math, you didn't study when you were a student.
ーNo wonder you didn't do well in math, you didn't study when you were a student.
「算数ができないのもそのはずさ、君は学生時代勉強していないもの。」
no wonder で「それもそのはず・道理で・無理もない」
ーYou didn't spend time studying math at school, so it's not strange at all that you had trouble with math.
「君は学校で数学の勉強に時間を費やさなかったから、数学が苦手なのも全然不思議じゃない。」
it's not strange at all で「全然不思議ではない」とも言えます。
have trouble with ... で「…に苦戦する・手を焼く」
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